I *finally* got the 'seeking' working. When the video is playing back, the seek bar at the bottom of the control page shows the current position in the video. I thought I'd also use this as a draggable control to let me position to anywhere in the video. Well, that turned out to be much more convoluted than it should have been, because of the tricky interaction between the playback interrupt and the 'buffer stuffer' interrupt. It finally boiled down to a calculation that was screwing up, but it took me ages to find.
Note that there is a bit of a delay between when you seek to a new position, and when the data becomes available to play - during which the disc is slowing a bit, so the system needs to resynchronise after each seek, so the frame "wobbles" for a few seconds while that's happening. I'm quite happy leaving that as it is.
You can also see yet another title screen - I seem to tire of whatever is there after a few days.
I think I've now fixed most of the bugs I know about. I really need to move on to the hardware consolidation, and then some sort of enclosure/cabinet/stand.
youtu.be/Ox3V3sJd9D4