Compressor accepts annotations on the command line via a plist file. This lets you add things like keyword metadata, author, etc. But Apple has neglected to document the file format. Luckily it wasn’t too hard to reverse engineer by poking at the traffic the compressor GUI sends to qmaster.
Under the root node, just make keys named ‘com.apple.quicktime.keyword’ or .producer or whatever. Then populate the values as you wish.
Attached is a sample plist file with a producer and keyword field. You’ll probably need to ‘view source’ to actually see it in your browser. Merry Christmas.