Maybe the way to go is to just use iMovie/iDVD to create a DVD disk image, and then break it up into individual files by splitting up the .vob files at chapter boundaries, extracting MPEG-2 files from the DVD data that way. Then I'll only have had to encode each video once. Are there any tools to do that, for either OSX or Linux?

Meanwhile, I'm playing around with the "Export" settings in iMovie and trying to find a way to end up with an MPEG-4 file that is roughly equivalent to what iDVD is putting on the discs (in terms of size and quality) and I haven't found the right setting yet. I wonder what frame rate iDVD uses? Anything less than 30 (well, 24 and 15) has really visible field-splitting artifacts that don't show up in the DVD version; but those files are around 150% the size as they should be. (I think I should be ending up with roughly 53MB/minute, right? Because a DVD is 4.7GB, and iDVD writes 90 minutes max?) An export quality setting of "High" (75%?) seems ok, but "Medium" is crap. I'm using the default key frame rate of 90, and turned off data rate limit.