Somebody mentioned Listerine! I've a set of hard 500 carb boots and a big, Sams club sized bottle of Listerine. I added 1.5 cups (about 12 oz) of Listerine to about 0.5 gallon of water, and set the pot on boil! I'm reducing to a light simmer, but even after only 5 minutes there is a huge difference! I'll report in half an hour or so.
EDIT: OK, so I left them simmering for about 40 minutes. Over that time, I probably added another cup and a half of Listerine and a couple cups of water. The boots are far more supple than they were before, sliding onto the passageways much more easily, and bending under my fingers. However, I think they could still be a bit better. I'm going to let them sit in the formula at room temp. overnight. I'll check on it occasionally, but the Listerine is only 0.06% Methyl Salicate. However, it's also only 0.092% Eucalyptol and 0.042 Menthol, but the steam from that simmer sure did clear up my sinuses!
The wife was on the other side of the house, and she didn't like it one bit. I think she was overreacting, though.
So far, the Pros: Supple rubber, didn't cost me anything, good for the sinuses
the Cons: Complaining wife
Will update again later!
EDIT: So after letting them sit at room temperature for 12 hours, the boots have LOST some of the suppleness that they just regained. My guess is that the heat did more than the minuscule amount of methyl salicate. FWIW, I ended up using about 3 cups of listerine, which equates to 1.44 oz of methyl salicate.
SO THE RESULT: Didn't do much, wasn't worth the hassle. I'll probably go out and buy a bottle of wintergreen and do it right.