I have a fixed/fixed flip-flop hub and the lockring threads are stripped out on one side. Would I be ok to just thread a single-speed freewheel on that side and use it as a fixed/free hub?
Yeah it will work without issue.
Humans would have to generate far more torque than we are capable of to actually pull the threads off of the hub. If you think about it, if it's enough threads for a track cog then there's no reason it wouldn't be enough for a freewheel. Due to the smaller amount of engagement on a fixed thread the mechanical stresses would be slightly different but once again not enough to matter for our power output.
I can tell you that I've run a couple of setups like this for years, as have many others, with no issue on any of them related to the topic at hand.