> Steve, I gather you don't think it's a problem to use arbitrarily named tags on the command line. The launcher shouldn't look for an "h" tag for `py -h`

Sure, we can reserve some tags - the -x/y syntax would let you refer to a conflicting one. But other than PythonCore\x.y for (x, y) <= (3, 4), there shouldn't be any need for version or binary checks. Those were needed because we didn't write correct tags, but as of Python 3.5 we write good tags and PEP 514 essentially requires it.