Is there a way to have dashed barlines to join an ossia staff, except where there are rehearsal marks?
Short answer: no. But you can choose via Layout Options whether or not system items like rehearsal marks should go inside or outside ossias by default. I believe the default behaviour is now for them to go outside rather than inside in any case.
Is there any chance that things like rehearsal marks could be made to have opaque backgrounds to enable this kind of thing?
Yes, in theory rehearsal marks could be drawn with opaque backgrounds. I’ll make a note of it.
Thank you.