[Espen Dale Andersen]"OK, so I have a ProRes4444 QT clip with a luma key (alpha)"
That sounds confusing and contradictory.
Luma and Alpha are two different ways to get transparency on a picture.
Can you post a still of your Prores 444 file?
[Espen Dale Andersen]"I want to isolate the alpha channel and export the clip as to two separate files, FILL and KEY, to two ProRes422-files in QT-wrappers.
How do I do this? I have spent an hour searching, but all I have found are tips on how to export files with alpha included in the file. That is not what I want.
I can view the alpha alone by going "View->Channels", but I do not know how to export it."
Proress 422 do not support Alpha channel.
You can export a B&W matte to be used lately as a "Luma matte".
Put a black in V1, the picture with the Alpha in V2 and a white in V3.
Set the Composite mode of V3 (white) to Travel Matte Alpha.
You will get a gray scale movie that you can use as "Travel matte Luma" to pull the key on other picture.
In the mean time I made it work by using the filter "Channel Swap" under Channels in FCP7. I swapped all the RGB-channels with the alpha channel and exported. It did, however, lead to a long render time so I suppose your way is a better one.
Thanks again. Now I can playback the fill and the key separately from my plaback server.