No point in borrowing trouble. Let’s wait and see on this one. I have to assume that somebody tried it!

This is mentioned in their announcement :

Additionally, Finale becomes the only music notation software that allows for both SMuFL and non-SMuFL fonts. With this new feature, Finale v27 is set to become the leader in collaborative creation of effortlessly-readable music notation.

Lilypond can use both SMuFL and non-SMuFL fonts.

I think you can rely on MM to making sure that old file open correctly (as the first priority, above any other development). 

New documents will default to SMuFL. Old ones won't. You'll be able to convert from one to other with a script and a Library file.