Treat the RM as if it was just another part of the core.
You can create addons that extend the RM (just like you create addons that extend the core) but to use them someone would need the RM themselves. You can't just repackage the RM with your own extra code, just like you can't modify the core and release the whole thing as your own.
You can of course do whatever you want to your own copy of the RM (assuming things like copyrights, etc aren't removed), just like you can do whatever to your own copy of the core code.
Edit: The above is my interpretation, not anything official I've seen.