Easy-off oven cleaner is quite good, or so I hear. It comes in spray cans and is a sort of foam that should cover the minis for a good hour or two before cleaning them.
I have stripped many a mini, both metal and plastic with DOT4 brake fluid. Just let the mini soak in the stuff for 24 hours, then brush off with an old toothbrush under a running tap and presto; all clean.
Take care though; the stuff is sort of toxic (just like the oven cleaner), so I advise against getting it on your hands (I use disposable plastic gloves (not rubber) I bought in a pack of 100 pairs at the local DIY when handling the soaked minis. The rinse under the tap will clean them of any residu, but I always wash with dishwash liquid afterwards to be sure.
I also heard that Dettol, when used too long on a plastic mini, will make the plastic bendy/rubbery, but this is hearsay; haven't used it myself...