The Menu key is not used as a modifier in Windows or in any desktop environment I have tested - it just causes a menu
to open on key release. The Fn key is a modifier, however.
Therefore, I think that the Menu and Fn could be overloaded on the same key: If you hold it and press another key then it should work as Fn-layer modifier but if you would press and release it without pressing another key then it should produce a Menu Down event followed by a Menu Up event in quick succession.
For me who uses Swedish layout, a keyboard without Alt Gr (right Alt) is useless. Without it, there is no @ or brackets for programming. Many more useful symbols on Alt Gr in Linux than Windows.