I know adding a ritardando feature has been on the list for ages, and I imagine this may need the same underlying code changes, but it would be really useful to be able to add a pause to a beat (so I suppose it would be like changing the BPM just for a single beat).
Although not weighing in on the pause, I have a related possible tip. Fermatas (lengthening the beat) are approximately twice the note value. I just double the note by adding, e.g., a Strum measure. I suppose -you could try half-measures of rest for the time being.