a strong caustic (sodium hydroxide) like a drain cleaner might work, if it does, besure to THOROUGHLY rinse with water afterwards. Then dry competely and oil. Take stock off first. Caustic will eat aluminium! be careful. Do not use an acid, acid will eat the steel. Of course, if you soak the patch with nitric acid, and then let it dry out, it will self ignite after a while.
A black powder patch worm (not thebullet puller screw) might work also, this thing looks like a pair of hooks that screws on the end of the cleaning rod. You may be able to dig into the cloth and pull it out, or tear pieces off until you can pull it out.
Good luck.