Although I don't use Macs, I assume you can still run more than one track.
If so you can do this on the 2nd or more tracks.
Open Photoshop, Illustrator or Corel or whatever, create two black bands top and bottom,
to whatever dimensions you need you'll have to start with any background at 4:3 and then
figure out the dimensions for 16: 9, save this file as an image.
Then on your overlay track drop it in to your timeline and repeat as necessary or stretch out the time.
The black lines should literally act as a letterbox. Then when you create your movie file or bounce down
it should all come together.
I use a similar way to get full widescreen movie2.35:1 on a 16;9 edit.
hope this helps.