I have gone and done it! Experimented with the Da-Lite HP sample they sent me, and it works as I theorized...well sort of!
Tried the highlighter pen (pink, it was all I had), and just as I thought, the HP retained it's high gain, retro-reflective properties but now with a different shade!
Next I tried was dye. All I could get was black, but it turned out it was all that I needed. Mixed it with water, and applied to the HP and got a shade of gray. Again, the HP retained all it's properties, so much so that the gray half of the sample I made was not showing up at all when viewed as close as you can get from the PJ. Re-applied more dye, this time without any water mixed in (concentrated black dye). Left it on for about five minutes (the longer you leave it on, the darker it gets), and rinsed away the excess with water. Got a dark shade of gray this time, and still the HP retained all it's properties, and got more of the gray screen effect I was looking for(it was the same shade but slightly darker than the Da-Lite super wonderlite). Still, as dark as it was, the effect was minimal, again when viewed from about where the projector is located. It's just a matter of how dark you can get gray on the HP at this point, but now for the downside!
When veiwed off axis, especially outside the width of my screen, retro-reflectivity rears it's ugly head even more, for me anyway! The further you move away from the optimal viewing angle of the HP, the darker the shade of gray becomes. In effect, this exaggerates the downside of retro-reflective screens. Now, instead of the HP reverting to say a 1.0 gain screen when veiwed off axis, it becomes gray directly proportional to the veiwing angle.
Before I even thought about this experiment, the HP's retro-reflective properties after seeing it for myself, is what's really turned me off to it. Applying dye to get a gray HP has made it worse. But on a good note, this experiment proves it works. The next logical thing to do is to try it on the other type of high reflective screens out there. The non retro-reflective type like the one Draper makes, or is it Stewart? Anybody else care to try?