Are there any hot springs or any thermal features at all in Yosemite National Park?

I doubt it. If there were any I would think they would be along Parker Pass Creek or Granite Lakes, closest you get to the type of area/geology you have in Devil's Postpile, which does have hot springs. But none there that I know of.

I'm not aware of any either. I know of several hot springs off hwy 395, within 1/2 hr of the eastern entrance of Yosemite, e.g. near Mammoth. There are supposed to be springs in the Bridgeport area and on both the north and south shores of Mono Lake, all a short distance from the Tioga pass entrance, but I haven't checked them out personally. If you're in the area and really want to see volcanic activity, drive a few hours north to Lassen Volcanic National Park. Lassen peak is the southernmost cascade volcano and has erupted within the last 100 years. It's a beautiful place that gets much less traffic than Yosemite.