It would just be an overlay so no program is required and if you had the ability to drag and drop the scale you could just position one edge on the hole. The problem would be having the scale be relative to the camera view as far as scaling goes. Not all the aerial shots are at the same altitude so that would have to be factored in.
I was thinking more along the line of (1) click a spot on the green (say the 65 point), then enter 65. Click another point on the screen (say the 90 point), then enter 90. Enter the distance you want and the program returns a scale the size you've requested based on pixels. You then position the furthest end at the flagstick and then move your marker to the other end.
If it was WGT that was programming this, your idea would be perfect. They would already know distances plus where you are ... if you enter, say 16 feet left, it could just move the marker to that position for you. No need for a scale at all.
I'm no programmer, (wow understatement there), but would Pythagoras work? Most greens have 3 distance markers, connect them together to form a triangle and you then get distances. If you ensure you connect them using a right angled triangle you can calculate any missing distances using Pythagoras. From that could you work out the size of the green and then an overlay?
As to different views having different altitudes, that would be a problem but if there are two views that could be a benefit, calculate the size of the green twice as a double check, once you know the size for an overlay moving it off the green wouldn't be a problem (would it?) so long as it was still to scale.
Sorry if this isn't possible, just some brain droppings that occurred after reading your posts. Would be a good money maker for WGT for sure and should be easier for them as they (I hope) would have access to more relevant information.