Capo is basically a separate transpose of the same chords for guitars. So a separate flag for the song locator would be what I want.

i.e. "Song Name [capo+5]"

This way one ChordPro-formated lyrics-track would be able to output three versions: Lyrics only - Lyrics and chords - capo version

This would make it much easier to avoid errors. And easier to transpose quickly, as transposing with a capo is different from without. Playing with capo on fret 1 (song minus one), and transposing down 2 steps is not moving the capo to the 11th fret, but rather to the 5th fret and transposing the chords "song" minus 5.

Case study:

Normal song locator will be: "Song Name [+2] [capo+5] {D Major, Capo 5: A major}"

Resulting in this math:

Capo = chordpro track + song transpose + capo

Capo = C + [+2] + [+5] = A major

We now want to play in B. So I transpose the audio tracks -3

Songlocator will be: "Song Name [-1] [capo+6] {B Major, Capo 6: F major}"