Ok, this should do it.
The intention is to provide extra functionality to text spanners. By using this code, you have all the functionality of normal text spanners, but you are able to have multiple spanners per voice.
You simply need to swap the regular engraver for this one in a layout block, and use the commands I've defined: \startTextSpanOne, and the like. (You can change the names, of course, and add more. This is crying out for a better interface, I know,)
Regarding difficulties mentioned earlier in the thread:
The spanners should nest properly and maintain their orientation across line breaks. Here this is achieved by assigning minute variations of outside-staff-priority behind the scenes. (Turns out you can use outside-staff-priorities like 350.0001 ....) You can still override and tweak outside-staff-priority of TextSpanner if you want.
I believe that this manipulation may be avoided by inventing a new alignment grob "to bind them all." but I'm not sure. Since the attached seems sufficient, this could wait until time came (if ever) to add this to the code base.
DN
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text-spanner-id.ly