I'll probably end up being hated for this, but .... what purpose would this serve?
The standardization of using the various DIN sizes is what helps out a consumer, and there's various resources that have a list of what DIN size fits in what car. Knowing the DIN size of a car about to be purchased is useful if there's a particular head unit you want to use on a future car, or if you want to move equipment from one car to another. As long as the DIN size is equivilant, you're good to go regardless of the "face plate".
However, knowing the "face plate" dimensions of other cars but your own .... I can't see the benefit? I mean, I can measure my cars' and then work from that for what I want. But rarely would I imagine, "I know a xxxxxx car has an extra 3 inches above the DIN opening, so if I had that car I could do this." I'm more interested in what I have in front of me.
I can see that knowing if a 7" or 8" or 10.4" LCD screen will fit in the "face plate" area of a car as being usefull.... and there's already a great resource for that. The worklog / completed projects forum. Tons of examples, and although it's not every car it's a good deal. Besides, as well...... most people here MAKE the size of LCD they want fit.
Just being honest... perhaps there IS a reason, but I just don't see the benefit.