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.
I did try something like that by changing the time signature, but couldn’t get it to work. The piece is in 3/4 and the bars in question need to change chord and pause on the 3rd beat. Changing chord is easy enough by subdividing the beat. I thought that by changing time signature to 4/4 for those bars I could double up the final beat as you suggested, but even though I selected 4/4, SM only wanted to set it to 2/4.
Maybe: subdividing the 3/4 beat into its 3 beats; you get two with the chord and one you can rest. Then move on. That is best I can figure.