Have done some tests in FFMPEGX though which look promising. A problem with FFMPEGX is that it doesn't export into PhotoJPEG format ... just a lot of formats like DivX and MP4 ... nice for the web, but not exactly what I want to edit in.
Regardless, we'll see if it works.
Update: Now I'm trying MPEG Streamclip, which CAN export to PhotoJPEG and reads interlaced fields separately then reinterlaces them if scaling! Sounds perfect for this project. I'm verrry interested in seeing the results, as I think this will be the program that cracks this problem ...