[itsme]

I like Sciurius` idea of applying the '#' / `b` button to the chords within the song in case the current key (after applying transposition) is C. This should not be too difficult and would probably solve the issues of LD Jackson and Sciurius. I think I also came across a situation where I wanted to see a Bb chord within a song in C. On the other hand C C#dim Dm G7 is a common turnaround in C. So it really makes sense for the key of C to allow specifying if sharps or flats shall be used.
Editing the chordpro file is just a workaround that fails as soon as a song is transposed to C from a base key other than C.
For other keys I doubt that this is required. F is clearly a 'flat' key, G is a 'sharp' key. I can't imagine a situation where I would want to see an A# chord instead of a Bb within a song in F.

What will remain an open issue is the case when a song changes the key within the song. The base key remains unchanged, but a b-section or a bridge modulates to a different key. Or it modulates every verse a semitone up. I'm afraid there will always be exceptions that automatic transposition cannot handle correctly.