You will need to verify with neuron2 your dgindex comments, but I believe they are correct...maybe PM him.
You are right there is no way to test for sure what colorimitry is, but I assume all HD sources follow "the rules". In any case I always do this for my HD to SD conversions and my anecdotal evidence, at least on my 50" Samsung plasma, is very compelling.
Remeber, this is an add-on feature to your great program...probably best just to have two checks/buttons to add this. Then it is up to the user to change.
Thanks again for the consideration of this feature...again I am not a programmer so I can only complain LOL!!!!