Just found this:

http://web.me.com/andymees/Free_and_...tterbox_2.html

Christopher: ASSUMING the mock 16:9 video will never be over another video layer (ie. exists on V1), you could just go into the Motion tab and crop Top and Bottom by a number that gives you your chosen aspect. Write the numbers down and repeat OR take advantage of FCP's Paste Attributes function and apply enmasse to all the clips that need letterboxing.

If you're working with progressive footage then I would suggest setting the project as a 16x9 video then scaling up your clips to compensate for the additional frame width. Not only do you get the benefit of a more natural letterbox effect on standard 4:3 televisions, but it's already conformed to the correct aspect ratio for widescreen/HDTV sets.