I remember reading years ago that while Backbone was coded in Amos Pro originally, but the author either started to re-code parts of it or add non-Amos code, if that's possible. I've heard that ASM code can be added to Blitz games, perhaps the same can be done with Amos Pro? I don't really know the first thing about coding, sorry.
I do have a feeling that reverse engineering Backbone may be more of a headache than coding a new game maker from scratch, and using a more powerful language to start with, having AGA support from the start.
As for a tutorial, I'm pretty sure one is provided with the Backbone package, however if I have some free time this weekend, I might see what I can do.