I hate to keep spamming your thread, but here are my results--the BD-SUP method mentioned above is a winner for that particular quirk, but the DVD method doesn't do anything about the jitter.

The DVD player I was able to find that had the jittering effect was one that seemed to always output for a 4:3 display. It was old and had no option to specify 16:9. And I noticed that the jittery subtitles were shifted down in the frame from where I'd expect them to have been on a 16:9 display, so I believe it was doing something unique to the subtitles because of the display aspect ratio, even though the video content was anamorphic and it dealt with that fine.

I do not know if the player has to be in 4:3 output mode for this bug to get triggered, but that seems like a good lead. Encoding the subs on the DVD as LB or WIDE didn't seem to change the behavior at all. And the shimmering is not consistent either--some lines are almost perfectly stable, others are a jittery mess.

Do DVD subtitles contain both embedded LB and WIDE variations? Is it possible the LB variant is screwed up and the WIDE one is fine, or that the LB variant is missing and the player tries to apply the WIDE one to the wrong AR somehow?