i'm trying to do the same. USG behind a RUTX09 in bridge mode. i copied the mac address of eth0 of the USG (WAN interface) into the bridge mode configuration of the RUTX09. after that, I can see the USG getting the public ip, and i can say i get -some- traffic through. Google seems to work fine, but alot of other websites don't work at all or things are so slot they time out! i'm really lost here seeing that happen. no idea how to troubleshoot that.
switching back to NAT mode on the RUTX09 works just fine, no problems there. But thats not what i want.