Combining YUV channels in Shake?

I want to combine the luminance channel from one (black and white) video clip with the colour channels (but not the luminance information) from a second video clip, creating a colour video clip from a black and white one.

The best way to deal with these channels seems to be within YCbCr/YUV colour space where I can keep the blacks and whites seperate from the colours. Viewing the b/w clip YUV channels in Shake, indeed 2 of the 3 channels are just blank.

I just wondered if anyone knows how I might go about combining these in Shake? Also, when I export a .mov file from FCP6 and open it in Shake, it has become an RGBA clip, even though FCP works in YCbCr as I understand it. Where is the conversion happening, and how might I stop it?