My minor contribution to this thread would be:

I've got a homebrew that's similar in nature to the AGP and my rule is this: whatever seasonal limit you place on position players, you use 3/4 of that for catchers.  So (just to keep this simple) if your rule is all batters with more than 5 seasons of 400 PA, then for catchers it would be 5 seasons of more than 300 PA.  This works out very nicely to give you about 3 catchers per 2 players at every other position.