Hello my dear Rockband fellow.
Your question is a little hard to be answered.
I can not even get the dedicated guitar to work.
Since 3 days I have tried to get this Guitar Pro Controller to work:
I have tried it with
- USBLoader GX v3.0 r1271
- Configurable USB Loader Mod v70 r78.11
- Boot from Disc (without any other USB device attached then the Receiver for the Guitar like in the Manual described)
Rockband 3 starts,Controller is connected and found in the Start Menu.
Then I choose NEW PROFILE with the "+" Key on the Guitar and thats it.No other Keys works on the Guitar.I can proceed but no access to the Guitar and its Options (as Spieler 4):
Maybe the Controller is "broken"....i do not know it.It seems to work for me.
This "great" thing works ONLY for Rockband 3.
So Rockband and custom songs is maybe cancelled for the next time.
Very,very sad of this......
I watched and read your great
Tutorials and
"unfortunately", I was already looking forward to it.
I am very sorry my friend,I can not help you here on your question about "cross-compatibility" on other Consoles.
Please keep up your passion for your outstanding work ! I really appreciate it,my friend. 
Thank you.
