Okay, not really a single speed, just a single cog, but if I'm putting an IGH on a derailleur frame, is there anything I can screw in the mounting hole for the derailleur so that the threads are safe in case I ever want to go back to multiple cogs? Or am I just being paranoid about thread damage? I don't really need a tensioner since the frame has sliding dropouts.

I wouldnt worry about it, just clean the threads of all the accumulated gunk if ever decided to install a derailer or tensioner. or simply tape off the hole

10mm 1mm pitch.