Yes. I just did this a couple of days ago. I have a DSL modem that's in bridge mode on the WAN side of my monowall that I wanted to be able to access on its web GUI. I followed the instructions here -
http://wiki.m0n0.ch/wikka.php?wakka=AccessingModemOutsideFirewall - (the 2nd option - "Using PPPoE or PPTP for outside IP address") and it worked like a charm.
..Steve