if I input q=ca. 122 it enters, but the q stays a q instead of a note head. This is a fairly common thing in newer music and would be nice to be able to do.
write q=122, and THEN, once it’s on the score, select it and use the properties to add what you want 
Marc: thanks for the suggestion, but if I single click on the metronome mark nothing happens. If I double click on it, it goes into note entry mode with the carat, so it seems I can’t edit it once it’s there.
Click on the metronome mark and look for its properties in the bottom panel (cmd-8) There you should find everything you need. I attach a picture to make myself clear.
I added the tempo text and value using shift-T, then I wrote Allegro tranquillo q=122 in the popover.
Hope it helps !
Thanks Marc, that clarified the process. I would not have thought to look there.
Why not just put the tempo you want? Believe me, performers will add the ‘ca.’ element, with or without your permission!
But if you want ‘ca.’, that’s how.