So, the mirror screen guard I was talking about has arrived.
As I expected (I finally thought about how it could work AFTER I fired off the purchase) It's essentially a well cut one way mirror tint sheet. So when there's no light from the screen it appears almost as reflective as a mirror, but when the screen is on it'll show through. With a little caveat being that since it does fancy tricks with the light the screen is tinted and thus a whee bit darker than usual. It'd be fine if the screen was set to a higher brightness, but on the Diamond I can set the screen brightness to auto adjust based on ambient light but because the sensor doesn't know about the tinted screen thus I now have a slightly darker screen than I liked. (wonder if there's a way to tweak this sensitivity)
Quality wise, the surface feels nice and smooth both finger and stylus manipulation seem to be ok. Though I think I need a bit more pressure to tap on stuff, maybe I should perform a screen calibration again.
Would I recommend this to other users? No! Now I know why this is cheaper than other screen protectors even though it has a special gimmick. Not everyone is going to be able to live with the fact that the screen will appear slightly tinted unless you're always using your device on MAXIMUM brightness. But if you'd really like your phone's surface to look like a mirror... be my guest!