> - I haven't seen other requests for it

For the record, an Extend custom action class is one of very few such classes I have ever written for argparse for exactly the OP's usecase, i.e., it is useful for any parser that should accept the same option multiple times, while accepting multiple arguments each time.
It *is* very simple to implement (and that's why I never asked for it as a feature), but only once you've understood custom action classes, which are not exactly for beginners.
All in all, I'm +1 on the request.