[sciurius]

PlayTab:  This is new to me.  I had a quick peek at the author's site, but couldn't see any real advantages over either of the other two options, but it might be easier to implement within MSP.

PlayTab is a personal 'invention' of mine. I started writing it in 1995 because I wanted chord charts but there were no tools that did anything even remote to that.

Obviously, there is only an advantage in incorporating any of these formats if transposition is supported.  Without being able to transpose 'on the fly', it's a waste of effort.

Exactly. This is the crucial point.

If I need to write a chord chart, I use Band In A Box.

Ah, good ol' Band-in-a-Box... I never used it to produce chord charts and if I may be honest, the sample you included lacks the elegance of compactness.

And it's expensive (at least it was, last time I bought it. And the upgrades... I never upgraded beyond version 12a in 2006.) I hardly used it since I generally do not run Windows.

Having said that: BiaB also has a wealth of songs already available, but its file format is proprietary and much harder to crack and implement. There seems to be a converter from BiaB (chords only) to iRealPro.