so I noticed that the Atmel chip on the display board can report its firmware revision (see here). This isn't currently used for anything beyond a sanity check during probe.
I was wondering if this can be used to distinguish screens that support the correct PWM backlight control (not only on/off) and those that can't, i.e. is there a direct match to the board revisions? I don't have an older board around so I can't just test this. Currently the "failure mode" if you assume it's always a new display is that you can't see anything on the old ones, and the semi-logarithmic scale for the backlight intensity (as perceived) means if you restrict to >128 PWM values the brightness control is no longer very effective.