I'm fairly new to the game (my main is a 30/29 sorc, so i've yet to reach T4), maybe my input won't provide much you don't know already.
Still, out of my perspective (the follower's perspective) it appears, that it not only/primarily depends upon your person/leadership whether or not people will follow your orders. For a random warband, at least to me, it's far greater a problem to trust the other players than it is in trusting you.
Let's take the example you already gave: You order a single group of people to defend a BO, while the rest of the warband is active elsewhere. Now, if I'm in that group, a number of concerns will arise.
Concerns like: "If we are attacked, will our tank just happily pound on the enemy's while I'm getting slaugthered
by their mdps and our healer is running away?" - or maybe, while we are standing at the silent BO, your main force gets wiped in a major battle and people will leave as a result. It having been a major battle will have netted everyone
involved at least some rewards. I will get nothing. And so on.
As I'm a pretty team oriented player (I think) and don't care too much about rewards, I will comply anyways; others
however will not. Like I mentioned the main problem here is, that people won't act in any reliable fashion.
Your authority as leader to a certain degree remedies this. Since your word has more weight, it is more likely people will act on it than not. The more complex the order becomes (in this context splitting up is already pretty complex) and the more individual gain people will have to sacrifice for it, the less your authority will mean in comparison, as there are just too many random things that could go wrong in the process. If you have had success before, of course people will have more confidence in even a more difficult plan to succeed as you have shown that you can pull it off. But you don't enjoy such high trust from the start and, since even the best decisions can yield bad results through bad luck or a generally very disadvantageous situation, can't necessarily expect it to last.
So, while it is only very theoretical and based on my relatively narrow experience , I try to give some factors which I think could help build the trust needed to even follow a risky order.
First - this is basic stuff, you surely already do this - it seems to have a more positive effect on group morale (and therefore compliance - since people act out of their free will alone, not out of duty) to communicate your plans in the necessary detail; if needed give the whole plan from the start and possibly a reason why you are doing it this way. The more thought through the plan appears, the less it seems susceptible to random failure; so people will follow it more readily.
The second point is more crucial and ties directly into this. The most positive experiences I've had so far were with wb's, where there were multiple people who could have taken the lead - they were discussing what to do next, advantages/disadvantages etc. . Having a "core group" of (optimally) 5-6 people who are not only disciplined, but also communicative themselves will help greatly. You can put some of them in the group you want to send to the BO, the other spots you fill with randoms you think suitable. I have been in a similar situation where one group was dispatched to do an ungrateful job. One of our healers first wouldn't come, but, since multiple members of our group asked where he was and whispered him, tagged along in the end. It wasn't the wb leader who made him move, it was
the group itself.
So I would try to make friends with this sort of people; create a "pool" out of which you can maybe take like 3-4 people reliably to form your "command staff". This is similar to the earlier suggestion with the semi-guild-semi-random-wb. Only you don't need a guild group ready and connected. Having guildmates for this job
of course makes this easier (Make sure to still communicate in the wb chat for everyone to see).
Also if you notice people following orders disciplined without communicating much (like me

completely handpick the warband like it was already suggested. It's sufficient to have a core who are disciplined, because as soon people start noticing that they can rely on their comrades they will more readily follow the plan themselves.
Also, out of personal experience, PAY ATTENTION to what the people are doing/where they are. I once was in a WB in T2, where I got ganked multiple times, had difficulties reaching the keep and so on. We didn't regroup once, I was constantly isolated, dieing; nobody gave a ****, even though I was my group's healer. Result was: I stopped healing as I felt there was no support; so i wouldn't give any in return. When the enemy broke the keep gate; I ran away, while the rest of us wiped. Well half of us, because the other half couldn't make it to the keep in the first place.
Our leader didn't react to the situation at all. Maybe it's my personal opinion, but if the wb is simply moving from point A to point B and suddenly the HP of one member are dropping - there should be a reaction. It's the same for many similar cases. Being apprehensive of the wb's situation and, again, showing it through communication will rise acceptance for your orders because people will feel more connected to them.
In that sense, I second the suggestions of kicking people who aren't contributing to the wb.
While it may appear a bit harsh or even tyrannic on the one hand, one the other it shows the people remaining that you are recognizing what they do. They see that your plans and orders react to and change with their personal situation. Which in turn builds trust in your leadership.
Also, if you kick, kick quickly. Of course you should give a fair warning; but after that the response should follow immediately, elsewise it lowers the impact.
Having written all this, I find that my propositions just iterate what has already been mentioned. Still I hope you find it a bit helpful.
I'm interested in this topic as well, since even from a simple member's perspective it's quite frustrating when actually sound tactical orders fall victim to the herd mentality.
And finally: I have a question as well. Namely, how should a wb cope with an order, that by general consensus seems bad? Most people don't write in the chat, some may not even read it (Kick them!^^). So even if someone basically speaks out what everyone is thinking, there is no possibility to have a poll to change direction.
If the leader insists on his course, are the only possibilities to either leave the wb or follow regardless? Even though you yourself think it's a bad decision and everyone else thinks so, too?
I had this situation last night. Me and a couple of other guys did follow, at least half of our people ignored the commands entirely; I think the reason was that they didn't agree with the course, but somehow lacked a way of communicating it properly.