thats really interesting, could you put a thermometer in and take a reading? ive assumed that the carbon-nitrogen ratio would be too low with hugelkultur to generate much heat.
i would think that unless you are getting 150degf plus, your squash will be fine.
160-180deg f is enough to sterilise a good compost heap.my last proper, quick heap hit 160 just for a few hours, that was watered with sheep dag stew!
it would be interesting to see the progress of the rot into the wood at its core.