To try and narrow down the issue, I now have separate patches for an explicit sys.meta_path and a sys.path_hooks. Both fail on tests, but for different reasons.

The explicit sys.meta_path is still failing on the __main__ issue on three of the tests and for threaded imports on the other.

The explicit sys.path_hooks patch fails on two tests: one on threading and  then test_cmd_line_script on apparently lacking an absolute file path thanks to issue #8202 (and why an explicit sys.path_hooks triggers that I don't know).

Both failures Nick might know about.