Thanks, Drone - [HTML=https://forums.totalwar.org/vb/showthread.php?p=1121856]found it[/HTML].

It's for VI vanilla only, but there's also some discussion on it in [HTML=https://forums.totalwar.org/vb/archive/index.php/t-78061.html]this archived post[/HTML].

Those threads raise some interesting points, notably about the Muslims getting crusade-spammed if every catholic faction can crusade. Upping the tech level of the chapter house and the cost of crusades might help this (and also prevent me sending my early crusades - I always make Chapter House a priority for one province), but personally I'd rather see some other kind of limitation - a minimum number of provinces would be nice.

I might give this a play...