What kind of willwood set up are you trying to use?
I ask because i put a willwood disc brake set up on my 8-3/4 with the same axles but mine were 35 spline if that matters?
but i didn't haven any problems installing and tightening the axles flange nuts.
Also my disc were a drag race set up.
but to answer your question i don't see why you couldn't drill a hole, but i don't know if it would throw off your balance any?
also not to sound like a smart *** but have you called moser and ask them if you can drill a hole?