Yes, this solution occurred to me as well, and in fact the postprocessing that THS does would effectively separate the stdout (test result lines) and stderr (diag lines) as long as the test script conformed exactly to the TAP protocol, but that would entail modifying THS itself, a CPAN module not written or maintained by me, something which I am not willing to do at this point.

I was hoping there might be some way to redfine/manipulate STDERR in my script in order to capture it, but I suspect this will not work, since the test script is running in a separate process.