Not sure why this is so hard to find out, but I am having a heck of a time finding out whether or not Tomato USB supports the simple IPSec passthrough that my old D-Link "g" router supported. It was just a checkbox on the D-Link that when checked allowed any connected computers to use my company's proprietary (AT&T actually) VPN that uses IPsec, and everything just worked. My old g router supported "IPSec Passthru", nothing fancy. Even my far old "b" access point (now in a junk drawer) supported IPsec passthrough and worked well with my VPN.
I plan to put Tomato USB (either VPN or EXt, whatever the last one is) on a new Asus RT-N16 router that I bought yesterday. I don't need anything fancy for VPN support, just the simple passthrough of IPSec that has been around in routers for many years. Running OpenVPN on my work machine (only computer I have that needs VPN support) is out of the question. For this, I just want Tomato/Asus N16 to "just work" (and be stable) with the only VPN I am allowed to use.
Will it work?