Gould recommends using a time signature font for repeat endings (p. 236), and I’ve seen some publishers do this as well. This functionality is already available for fingerings, so would it be possible to enable this for repeat endings, too?
This is an interesting idea. It’s not completely straightforward to do because Dorico allows you to input any text you like in a repeat ending, and the repertoire of glyphs in the bold style used for time signatures is rather limited, so there would have to be some kind of fallback behaviour for when you start adjusting the text beyond the regular numerals. I’ll think about this for the future.