Router in half-bridge mode

We have an odd-setup at one of our branches that I am hoping somebody has run into before. ATT had provided us a modem/router setup in half-bridge mode, where we provided the PPPoE authentication on a PIX firewall and assigned a static address to our PIX. Unfortunately it appears that this kind of setup is not available in the Balance 380 where it is a static IP or PPPoE WAN link. This is a weird configuration but I do have a couple sites with this same setup. Any suggestions will be appreciated.

just an update. the recently released 6.1 firmware resolves this issue. Thanks Peplink.