From John C:

Currently, if you schedule a message for sending you cannot use that message to mark the thread as "resolved" until the message has actually been sent. This is intentional as, until the message has been sent, other users can't see the message. So if we naively allowed marking a scheduled message as the "resolution" of a thread, other users would see that the thread was resolved but they wouldn't see the resolution.

Probably the best approach would be to allow marking the thread as resolved immediately, but other users wouldn't see the thread as "resolved" unless they could also see the message marking the thread as resolved. Though this could make search tricky if we allow filtering for resolved/not resolved threads.