Many craft stores sell extra-long cotton swabs with wooden handles. I suggest depressing the magazine follower with a non-marring tool such as a wooden or plastic dowel rod, then digging out the crud with swabs soaked in powder solvent.
Another option is non-chlorinated brake cleaner, which is available at auto parts stores and the automotive section of big-box discount stores, usually in a green can. (Do NOT use chlorinated brake cleaner, carburetor cleaner, or fuel injector / throttle body spray cleaners; they are harsher and may attack the plastic follower and base plate!) However, I would try the swabs first. Brake cleaner will be smelly and messy, there may be an issue with the fluid not draining freely from inside the magazine, and the mag body will have to be thoroughly re-oiled afterwards to prevent rusting.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak