Can I make rehearsal marks with arbitrary text?
You can set the first letter via the Index property in the Properties Panel. Then use the Suffix Property to add text to it.
I’m not sure of your use case, but you can also just use System Text. If your default System Text Paragraph Style doesn’t include a border, it’s easy to add one in Properties:
You can also duplicate your System Text Paragraph Style, configure the enclosure, save that as a style, and select it as style when you create the System Text. System text won’t require you to fix the rehearsal mark index of the next rehearsal mark either.