Scale effect

I don't think that overspraying with white would yield the results you're looking for. In all the serious discussions of scale effect that I have seen, the recomendation is to mix in white paint with the color prior to airbrushing. Considering the effect that lighting conditions and weathering have on the perceived colors of a vehicle, plane, or ship, I don't overly religious about "accurate" colors. I try get colors that "look" right and don't worry much beyond that.

I understand what you are trying to do, but I don't think I'd want to spray any percentage of white paint over a paint job I spent a lot of time on.

What you might want to experiment with is this:

If you mix Tamiya XF-21 Flat Base with Future you can control the amount of matte finish. About 5:1 Future:XF-21 gives a sort of satin finish; 4:1 is pretty flat; 3:1 is dead flat. If you get too much XF-21 it starts to cause a white haze which, I think, is what you are looking for. You might play around with that mix starting at about 3:1 and working toward more XF-21 and see what works. XF-21 is NOT a paint, it is a flat base and should NEVER be sprayed alone, but it does have a white tint to it.