OK, never mind, I figured it out. The only points of conflict between my relay and the TBB are that the control ports and Socks listener ports for the two Tor instances both default to the same values. So I changed the ControlPort in Data\Vidalia\vidalia.conf to 9151 and added SocksPort 9150 to Data\Tor\torrc, and now the TBB runs alongside my relay without any issues.
It might not be a bad idea to change the default ports of one bundle or the other so that they don't conflict out of the box any more, or at least mention the need to change them in the applicable FAQs.