Did you ever solve this? Backlight flickering when you access memory above 5MB? I would doubt that the lack of a /DTACK signal has anything to do with it - if the CPU needed /DTACK but it wasn't present, it should just freeze up, or else there might be some external logic that forces a bus error. Either way I think you'd see major software problems, not just a flickering backlight.
My guess is you may have bus contention. Something else is mapped into the memory region above 5MB, and now your RAM card and that something else are trying to drive the bus at the same time. This causes a sudden increase in the power supply current, making the supply voltage droop, and causing the backlight to visibly flicker. The only problem with my theory is that if you had bus contention, you should also also see noise/bad data on the bus, leading to software errors. Maybe the new RAM chips just have more powerful drivers, and overwhelm whatever other thing they're fighting with.
Very nice work!