I think you may be out of luck...
The Microsoft Wireless standard on the Xbox 360 is proprietary so your not gonna find a 3rd party solution for extending your controllers. Heck, you can't even find a 3rd party wireless controller. As someone stated above, Red Octane is fighting with Microsoft to make a wireless guitar for Guitar Hero II and not getting very far. You cannot synch up a controller to the Wireless USB Network adapter. That is for 802.11b or 802.11g network connectivity only.
Maybe as more and more people install AV distribution system in their homes, Microsoft will see a demand for providing a solution to extend the wireless controller range.
The truth is 99% of 360 owners have their consoles in the same room as the TV it's hooked up to (or at least within the 30ft max range of it).
I have the Wireless USB Gaming Receiver for Windows but never thought of trying it on my 360. I'll try it tonight and see if it works, I'm guessing not though. I think it is PC only. But being that the receiver has a fairly long cord (~3-4ft), if it works, that might be enough to get you within range.
Exactly how far away is the back room from the 360?