From my experience smokelss can be used in a muzzleloader but not in the way they sell it.
I have used nitrocelulose made into a solid charge by a friend. You can look at my thread:

http://forums.nitroexpress.com/showflat....true#Post101754

IIRC IMR is going to produce a smokeless solid charge in 2009, White Hots is the name.

Hope this helps.
Martin