@mary-freiberg - to my knowledge it can't be specifically done yet - however, there might be a back door to accomplish this...
If you change the reservation terms to new terms (create a new quote with identical terms and agree to them for the guest), then I think the outbox refreshes. If it doesn't work doing that, try changing the arrival date, and then changing it back again (requires two terms changes). That might do it? I suggest that because I've found that when I add a night for a guest, it sometimes re-populates messages in the outbox that I've already sent.
Let me know if that works?