Given the intended use, I don't see the need for anything beyond any router w/ client mode capabilities here. dd-wrt or freshtomato (provided it has finally solved its client mode issues) is just fine.
On a side note, I find the "how to" a bit overly complicated. The author configures the operating mode as Router (thereby disabling NAT), then applies his own NAT rules to "fool [the] inverter that the routed packets comes from the DD-WRT’s IP address."
Uhh, unless I'm missing something, had he left the router in Gateway mode (NAT enabled), all the inverter would have seen is the WAN ip of the router!
At that point, beyond disabling its DHCP server, he could have given the router a LAN ip on the same IP network as his primary router, patched it LAN to LAN wrt the primary router, and added a static route to the primary router that points to that LAN ip as the gateway to the inverter's IP network, thus making the inverter accessible from any LAN client.
All in all, pretty simple. And frankly, I don't even think this requires a router w/ third-party capability. Just so long as it supports client mode, oem/stock or third-party.