Tried it but no success:

Code:
	unit Pilgrims			exp 0 cost 50 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Crusader Knights		exp 0 cost 400 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Crusader Sergeants		exp 0 cost 220 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Flagellants		exp 0 cost 150 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Religious Fanatics		exp 0 cost 110 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Great Cross		exp 1 cost 500 replenish 0.4 - 0.5 max 0.999 initial 0.5 religions { catholic }
	unit Ghazis			exp 0 cost 190 replenish 0.6 - 0.7 max 0.999 initial 0.5 religions { islam }
	unit Mutatawwi'a		exp 0 cost 100 replenish 0.6 - 0.7 max 0.999 initial 0.5 religions { islam }
Added these lines for each region listed in the descr_mercenaries.txt and started a new game as Hungary, calling a crusade and attacking the rebel village nearby with auto_win defender.
Couldn't retrain any of the crusader mercs both during and after the crusade and not after 10 turns still.

The only difference between my code and the default is the cost of the mercenaries, but that shouldn't make a difference, should it? I'm thinking that doing so just creates a second unit pool in the merc pool that is slightly higher priced and available to noncrusading factions too.