Rock Band VR v1.01 supports 360 guitars, it seems to not care who made it. I've only tested this using a 360 wireless receiver for Windows and a standard wireless 360 controller. In theory you don't even need this for 360 Guitar Hero controllers unless you want to remap some buttons.
The solution above is easier and safer, but if you're interested in what I was trying before the 1.01 update then read on.
ProxyGuitar is a C# program that attempts to emulate a USB/IP server that has a PS3 RB1 guitar - which it'll fill in the button data from a connected USB device (such as a Wii RB1 guitar not supported by RBVR).
Lag added shouldn't be that noticeable but the software is still being tested.
WIP attempt at a button mapping profile creator. Click the byte, click the bit then click the proxy button to map it to. Pressing buttons on the controller will light up the bytes/bits and any mapped proxy buttons.
I didn't get a chance to use the profile with the server, so it'll still be using the hard-coded Wii controller.
Source code in abandoned state: ProxyGuitar.zip
If you want to preview the code: https://github.com/Jas2o/ProxyGuitar
Includes an altered HidLibrary for faster device report reading. It has some leftover code to act as a man-in-middle with another USB/IP server (such as one running on an Android phone) in order to research how it would respond to certain USB requests.