Proportional, monospaced and metrics keys

I’m working on a project that has a monospaced axis. At the moment these are two separate files. I’d like to have all these masters as one file, but what’s stopping me are metrics keys: the proportional masters have lots of metrics keys while mono of course don’t have any. I can’t think of any simple way to merge the two except adding == to all mono glyphs but that would seem very annoying to work with.
Am I missing a potential solution, or should I just stick to different font files until production?

I would stick the metric keys at the layers (adding ==). A script can do that. A script that keeps them in sync would be handy, too :wink: