I have now completed the changes that you requested and pushed them to git.
They will show up in the next released version of LilyPond (or you can build
from git if you want it right now).
You can use verbose format for either addChordShape or
storePredefinedDiagram. Please note that you need to add a tuning indicator
to addChordShape (the new format is \addChordShape #'symbol #tuning
#diagram-descriptor).
You can also use (capo fret) as part of a verbose marking list. It will
create a solid line across all strings at fret. It will also make fret the
lowest fret in the diagram.
Thanks,
Carl