filter by class per-spawn room- possible?

You can't.
filter_activator_class refers to the game code, in which each different entity is a class, to say only allow npc_antlion to trigger something. It was around before TF2 and has nothing to do with player classes.

IIRC there was a way to force ppl to play a certain class by making the spawn room inescapable except by certain doors that upon entering the map would change the player to the appropriate class (using the point_clientcommand entity I believe) and then teleport them into the map proper. The idea is that each door is marked for a certain class and you choose your class by entering that door. You can't change class outside without dying and then being sent back into that room where you're stuck once again with the doors to choose class.

It's a little messy (and really frustrating that you still can't filter by player class, Valve!), but using this you could then decide where you'd like certain classes to spawn.