MM on linux is unlikely to happen, as it would be too costly buisness-wise. Maybe 0.001% of MM users would be interested in.
I think we would prefer to focus on developping new features than porting the app to another system.
app-to-app midi communication is possible either with a dedicated midi port or with a midi bus, such as loopMidi on windows, and the
IAC bus on mac (built in, just enable the bus in your audio midi preferences)