I finally figured out how to do the CXI_____ deals in LilyPond. They do exist. TextSpanner.
This would best be implemented in Rosegarden as an Indication, like hairpins and ottavas, that loosely associate with a note on either end to define their time, and already export that way. It would be a simple bread crumb trail to follow, except the bit where we draw the things in the GUI.
Hopefully I will get around to this while it's all still fresh in mind, and I don't need to spell out the details of the project with writing out all guitar notes with a script that I'm experimenting with to figure out the syntax for these things.
Not hugely difficult, good results, worth adding to Rosegarden.