Thank you for the research! I dug into the service manual a bit more and it describes the time interpolator chips (one main, one for windowed operation) as reporting back to the sweep controller chip, in 10 bit word form, the timing difference between the sample clock and trigger. Looks like this is different that the TDS544A. Maybe similar circuitry in the hybrid with an ADC?
The interpolators (165-2078-00) are hybrids bonded to a heat sink and screwed down to the upper acquisition board. Re-seating these didn't help, so I decided to take a leap of faith that there wasn't anything deadly wrong with the upper acq board, and transplanted one from a spare 602, and got it to pass partial EA. Also, the problem followed the original hybrid back to the spare, so looks like a bad hybrid.
Oddly, my 2nd 602 passed partial EA with the bad chip, but at 1nS per division, you can clearly see the single sample points indicating that the machine isn't interleaving samples..... going to wait and see what a full EA does on both...