I have taken an interest in this thread because some of my local guys run spec class, CS-27GL, but of course they are beginners and will crack the wheels often and the tires become useless. And for high school kids and novice, it's not feasible to buy a new set because 1 wheel is cracked or center broken. I'll check to see if the owner of the track will allow re-mount CS-27 and inserts onto say a more stout wheel, but it's definitely a lot more cost effective for these spec racers.
I have tried boiling for 1/2 hr and nothing is budging at all. I guess next step will be to try the oven method.... but I can imagine your kitchen will have a not so pleasant spell afterwards...
Dom