You might try a solvent like auto painters use. It's the solvent least likely to dissolve the rubber, and evaporates slowly enough to penetrate. Auto Zone, Checker, and the like usually stock it around the paint section. Naturally, try it on a very small spot of clean tire to see it it is affecting the rubber. I can't think of another solvent that wouldn't destroy the tires.
Some people are like a Slinky ... not really good for anything, but you still can't help but smile when you shove them down the stairs.