I can happily do that for you Aaron if you're able to produce the code to cycle the screen colours
Like you say, that's the only way to get the values without including the monitor although you may still need it to be done using two or three different RGB encoder boards because of the somewhat obsolete and strange component on there (we're taling about the board with the paper tube, yes?). I can borrow two or three of these little PCBs if you did want to check the results from more than one since another collector in the same town has a loft full of this kind of thing.
It would also provide an incentive to dump my romset too since i don't recall it matching either of the sets in mame from a point of view of not being either the hard set or the easy set.
It may not be very far different though, I've just never bothered doing anything with the cab since it's never needed any attention.