So much music (except for a lot of jazz) is in one key so I guess simple Nashville notation works OK.

I solved a similar problem when writing melodies in numbered musical notation (nmn) by using \1\ when a piece modulates down a step and /3/ to show it goes up a major third. My version of "Tune Up" looks like this:

Tune Up nmn by Miles Davis

Someday iReal b (and Player) will probably do something similar for Nashville and Roman numeral chords.