Yes, this isn't a hardware backdoor, it's all in the software.
This backdoor is a software backdoor - there may be hardware backdoors too. Hardware backdoors are much, much harder to find as there is no real way to track one down without trying to reverse the actual hardware itself, and that is close to impossible.
So while we can confirm there is a software backdoor in this router, we can't confirm if any other router does or doesn't have a hardware one.
If supported, I like Tomato (prefer the shibby variant) is my fav, a large number of routers are supported... I've been using and recommending a couple asus models.. RT-N12, RT-N16 and RT-N66(U/R) ... Been pretty good hardware (stock firmware sucks though).
Note: Tomato is imho a bit nicer than DD-WRT, but not as good for tweaking as OpenWRT (which I use on my office routerstation pro).
I don't know about the others, but I'm pretty sure that the WAG200G isn't supported by the free firmware projects because it's not powerful enough.