This would be similar to the Track Changes tool in Word. At a certain point the user would click «start tracking changes» and all modifications from that point on would be shown with a color or similar indication.
There are many times when mating surfaces need to be push-pulled by some amount to ensure proper fit and something like this would be helpful to ensure that we get everything and don't hit the same surface more than once (of course - some sort of automated fitting tolerances with ghost components would be better.. but that'd be a major new feature and this you could probably implement in a few hours).