However, if the threads are confined to the library's code only?
I think this should work, but there are caveats. Keep in mind the Windows port does create and use threads. However, the environment inside the condor process may not be what the threaded library expects, with respect to signals being blocked, uid switching happening at random times wrt the thread, etc.. Also, we have to be very careful that the threads don't end up in the syscall code that gets linked into standard universe user jobs.
_Greg