Hi Myles, came across on of your post about proxmox networking setup with 2 public ip subnet on same single NIC card. Looks like I have the same setup. I know this post is kind of old BUT wanted to see if you ever getting working? If so, I'd like to see if you can help me with my setup (would appreciated it!!!)
http://forum.proxmox.com/threads/12521-Networking-Setup-for-OpenVZ-and-Qemu-and-multiple-subnets
http://forum.proxmox.com/threads/12521-Networking-Setup-for-OpenVZ-and-Qemu-and-multiple-subnets