If you're using pppd to make your pptp connection (I've seen some that work this way ages ago -- don't know if it's still the case), then you should be able to use up and down scripts to deal with it.

Read your pppd man page and look at the end at the ``SCRIPTS'' section. You should be able to whip up a quick ip-up script that checks to see if $IFNAME is the correct one, and, if so, restart your firewall.