I think I figured this one out on my own…
I had to change my setup to advanced outbound NAT, create two NAT rules (one for each WAN interface), and make sure that the remote subnet was excluded from those rules. There is still no route to the remote subnet that displays in the web interface, but maybe that's normal. I'm just used to seeing one having come from a Linux/OpenSwan world.
So, judging from what I had to do, I'm assuming the NAT portion of the packet processing happens prior to the routing? It seems like you should figure out where the packet is headed before you figure out if it needs to be NAT'ed or not. ???