Okay, that was actually worse than I expected.

One idea I had was to subscribe Posterous (posterous.com) to the
Google Group, and then use the feed that they generate instead of the
Google Groups one. The only problem is that Posterous expects all
inbound email for a certain account to come from a strict set of
enumerated email addresses, which of course I can't guarantee.

It would technically be possible to simply subscribe people to both
services when they try to sign up for one of them, since after all I
am moderating the Google Group, but I think that a better idea is to
just let the feed be crappy. Sorry about that. If you don't like it,
then subscribe to the mailing list!