Right now too many factors can break the Wii. I would suggest that you start from the least restrictive setup and work backwards. Such as:
1. Connect the pcWRT to the modem directly, with UPnP enabled. Turn off parental control on the pcWRT at the router level. Does Wii work?
2. If Wii works, turn on parental control. Does Wii still work?
If both the above works, then move the pcWRT behind the Linksys and try the DMZ configuration.