I've read about a hotwater treatment but have never actually tried it. I hope someone who actually knows something will answer....

I haven't heard of anything new for Foliar Nematodes in the last two years. Treatment choices are (1) boiling water drench before crowns wake up (pips not visible), (2) NemaKill drench as crowns wake up (pips visible) and perhaps after (but best before hostas are fully leafed out) and (3) the rather drastic hot water soak that effectively kills the plant back to the crown but it resprouts (you hope) clean. (You can find instructions for this at Hallson's.)