Return of Reckoning

Is there any census data concerning how many of which class are either: online at the time, or rolled overall?

In your client.

Hit O at primetime.
And search for players but let the search field empty.
Will then display everyone online.
Group by career and you see the numbers. Well more precise the lines.
Do this on several times and you will get an overview in your head.
But nothing more Business Intelligence like imo.

As Glorian stated but there is also a good addon to help with that found:
viewtopic.php?f=66&t=17913