Subtitles in the letterboxed area?

[rumplestiltskin]

I have a 624x272 AVI into which I'd like to embed the subtitles. No problem doing this. However, I'd really like to put those subtitles into the black area that we'd normally call the "letterboxing". I've calculated that the 624x272 video would actually fill my LCD TV's screen if it was 624x352 so I'd like to put those subtitles into the "extra" 80 pixels of height.

I guess I'll have to add a black stripe to the bottom of the video. Could I possibly use a "120" or "130" in the subtitle position or will this just shift the subtitles out of the frame and not provide the extra black area?

Any advice will be appreciated.

Thanks

[Case]

I would only know how to do this in two steps:
- First encode to an enlarged frame size, to include a letterbox area.
You could add more black to the bottom, to make room for subs.

- Second encode to include subtitles.