Digging around a little, USB 5-pin Mini B extension cables (F-M) don't
seem to exist.

True, but take a look at this:

http://www.microcenter.com/single_pr...uct_id=0202234

Plug the existing hard wired male USB-B (mini) connector from the
antenna into this, then use a standard USB A-B cable from this connector
to the GPS.

I do confess that I did not haul out the manual to see how the XM
antenna is wired. So, the above is risk laden and guarantee free. As the
attached thread suggested, you may not be able to switch back and forth
from A to B USB connectors in both directions.

While I offer a possible solution above that I freely admit may not work
out, please save the sarcastic "BUT YOU ARE WAAAAAAAAY WRONG DUDE AND I
AM RIGHT!!!!!!!!!!" postings for other newsgroups.

Have fun,
Mike