Thanks very much.
A bit more info: The final video will be built in flash, so my final format would be .flv or more likely a PNG image sequence. The key doesn’t need to be perfect because it’s a silly internet promotion playing on the fact that it’s clearly a green screen shot. The 3 cameras will all be static. The best option seems to be to record footage and luma matte separately, bring both into FCP, punch it out there and re-export.