Thanks for your extensive answer. Many of the alternatives I've already tried, and they don't make me happy.

From an implementation POV I think this is a minor issue. It is not even necessary (although it would be convenient) to store the capo setting in the song file (this would require and extension of the current file format, with possible backward compatibility issues).

So I'd strongly appreciate adding this feature to iRealPro.
May I invite all other guitar players to support this request?