I was thinking through this and while I'm confident in my conclusions, I thought this might be a good topic for discussion and for players to have confirmed as acceptable.
It's accepted that it is not a violation of inalienable rights to give commanders more gold because they recruited whatever unit type you'd like to see more men of. Likewise, I would conclude that it'd be okay to give extra gold to characters for being a certain class and making that generosity known.
At the same time, giving out extra gold for courtiers, diplomats, infiltrators or what have you is manual and somewhat tedious compared to the usual military refitting and getting extra gold anyway process. So I would extend this hypothetical situation to accepting that there is a strong similarity of giving extra gold and changing estate tax sizes. Also, its made quite clear that the estates are the property of the lord and they may kick you out at whim. Thus, I don't think its too much further to conclude that it would be acceptable to have estates sized at a generous amount, but only available for nobles with certain character classes, as an incentive to more of those particularly desired classes.
TL;DR - is it an acceptable method to provide generous estates to nobles of certain classes and kicking them out (or reducing estate size if you prefer) if they are no longer the desired class? You aren't preventing them from playing whatever class they desire, only from having that bountiful estate whilst they are a class you don't wish to manage that estate.
