I just moved and changed ISPs. I got a Linksys router with DD-WRT on it, but my Roku Express will not see my wireless network. It can see other (neighbor's) networks though. Other devices see my router just fine.
Things I have tried (all on 2.4Ghz WiFi network)...
It looks like you've tried everything, but just make sure you're not overlooking something simple like ensuring SSID broadcast is enabled.
Sorry, yes. SSID is set to broadcast.
As you may have seen, there is/was an issue with OpenWrt and 2.4GHz and the solution was to disable "Legacy" (B) mode. It looks like you've already tried that. Updating OpenWrt to the latest version also seemed to fix the problem, but it could have been that the updated version just disabled legacy mode by default. Still, I'd make sure you're using the latest version of DD-WRT. One last thing to try, if possible, is to use OpenWrt instead of DD-WRT.