I'm very pleased to hear about the move to leverage the Mac OS ROM file via @ELN's tbxi project.

Yes, that's the way. I think literal system Extensions exist inside it?
Which brings me to the following point: @joevt I believe Old World ROM Macs will have to rely on a normal system Extension that would go inside the Extensions folder. What that might imply, though, is that USB booting with 2.0 speeds might not happen, but then again, I think no Old World ROM Mac can boot via USB at all, can they?
I'm not aware of Old World ROM Mac being able to use the Mac OS ROM file (although I could be wrong), but what I know they
can use is
System Enablers, AKA "gibblies". I don't know if it's possible to put USB drivers on them, though.
Worst case scenario, the app/service implementation we currently have could be kept for Old World ROM Macs if all else fails, whereas we leverage a Mac OS ROM patch for New World ROM Macs.