Generally, when the pcWRT is out on the Internet (such as being the DMZ host), and with UPnP enabled, the Xbox test should detect moderate NAT. Can you check these two items?
1. The pcWRT has a public IP address:
* load the Status page in your browser, look at the Internet IP address at the top left corner.
* Enter http://myip.dnsomatic.com/
The IP addresses reported by the above two methods should be the same.
2. UPnP is enabled on the pcWRT.