I couldn't see the wood for the trees:
If put() is thread-blocking, multiprocessing.Queue is reduced to the same functionality as multiprocessing.queues.SimpleQueue, if I'm not mistaken. So maybe there should be a warning in the documentation that, for multiprocessing.[Joinable]Queue, modifying obj straight after calling put(obj) might en-queue the modified version.
To me at least this wasn't obvious until I looked at the multiprocessing.queue code.
I've modified the example for clarity and retracted the (unworthy) patch attempt.
Regards,
VT