If you're using MFR anyway why not just grind all the mobs from your normal mob trap and then use the essence to spawn skeletons wherever you want them? That is what I did. I've got 4 5x5 cursed earth spawner/grinders dumping mob essence into an AE system. Then use an autospawner to spawn skeletons which conveyor belts dump onto spikes covered by a thin flow of witch water.
What did I need all those wither skulls for? Infusion crafting instability reduction. I'd put up some pictures if the auth servers were up.
I used the cofh killall command in the nether to get it to start spawning mobs properly.
Oh hey, now this is an Epic Reward.