I can safely say that you can't make money out of this platform anymore, the user base is just too small. If I had to live with the donations coming from my apps for BB10 well, let's say I would have already starved to death ;)
Not the original poster, but I have a hard time imagining you'd be able to meaningfully maintain the OS and integrated software (e.g. Hub, supported TLS versions for mail, etc.) via "mods" of any kind.
What _might_ be useful and actually possible would be an up-to-date maintained "rebuild" / "rewrite" of the native browser app, since there you can vendor pretty much all the important components you need.
I do not want to see these devices fall into obsolescence, and they are simple/old enough you could reverse-engineer the parts or get in touch with the asian companies who did some of the production runs for RIM.