For what it's worth, using a Perl script, ImageMagick, and an old version of BDSup2Sub, I think I've found a way to remove this jitter in a more-or-less automated fashion. I'm still checking for bugs but things are very promising (my first "successful" run introduced some color problems, and I can't be sure there aren't other surprises lurking in there)
First off, you made an observation in another forum that you can check a subtitle for this problem by opening it in DVDSubEdit, enabling the zoom window, and then adjusting the vertical position. If the image in the zoom window wobbles up and down one pixel as you adjust the position, the subtitle has this problem--otherwise it's good. This is a nice way of testing if you don't have easy access to a problematic hardware player (or an endless stack of DVD-R's :-/ ).
Second, the problem seems to be caused entirely by cropping. To fix this problem, BDSUp2Sub should not only avoid autocropping DVD subtitle formats, but should also actively UNCROP already-cropped subtitles if they are being output in a DVD format. Uncropping the subs nearly doubles the file size, so I can see why a developer may choose not to implement this fix for a problem that affects so few(?) devices. BDSup2Sub 4.0.1 (my preferred version because of its wildcard handling), allows the command line argument /acrop:0 which prevents cropping of uncropped sources, but it does nothing to fix already-cropped sources (and it doesn't work at all for GUI operations).
Assuming all goes well, I'll share my Perl script and instructions. They are designed to work specifically with my project and will need some tweaks before they can be a more general-purpose solution. For example, my scripts use 720p BD-SUP files as the source for the DVD subs, and all scripts assume the subs are widescreen. Also, this was a quick-and-dirty solution, so the complete lack or error handling and horrifying code passing itself off as an XML parser may need some touching up.
This is a busy week for me too so don't expect too much right now.