you should be able to run OpenVPN when tomato is not routing. Just have to port forward on the U-Verse Gateway instead. I used to do this with an SSH server running on tomato.
Alright, so I can understand that. Next up, I have some Cisco IP Phones which need DHCP option 150 specified. I've been doing this with the dnsmasq built into Tomato. How can I fix that if I rely on the U-Verse gateway?
If you don't have too many Cisco phones, just use the Alternate TFTP option in the network configuration on the phone and manually specify the TFTP address. This is what I do to any Cisco phone that I send home with someone, as that's all option 150 does...just using DHCP.