Sure it's possible, but I'm not sure how accurate they would be.
Most of the slugs and minnies I'm aware of are made to be loaded with just lube on them so they are pretty close to bore size before they are fired.
I doubt that those would easily loaded with even a .005 or thinner patch on them.
Patched balls are usually .005 to .010 less than bore size. When you add twice the patch thickness the combination is larger than the rifling size so they fit very tightly in the bore.
The area of contact between the ball and barrel is quite small so it is not real hard to deform it when you load. Even then, with a tight fitting ball/patch combination many of us find it necessary to use a short starter just to get the patched ball into the bore.
With a slug, with it's greatly increased length of contact and a tight patch it would probably need a hammer to get it started.
Of course that would probably mash the nose of the slug a bit so the accuracy would be poor.
Remember that the patch does more than just grab the rifling and ball. It also fills the rifling grooves to keep the leakage of gas to an absolute minimum.
That task pretty much rules out using any patch material that is thinner than the rifling grooves are deep, so, if you had a slug that was .005 under bore size and patched it with some .0025 thick patching it would fit the bore but it wouldn't fill the grooves to seal them.
The paper patch could be mentioned as it is used with slugs but, IMO they work well only in a barrel that is made for them.
A large part of making the barrel for paper patches is to have very shallow rifling grooves.
My Schuetzen shoots paper patched slugs very well, but the rifling is fast (1:18) and the grooves are shallow (.003 deep).
All in all, I don't think patching a slug in most current muzzleloaders is a practical idea.