The promo vid from Pioneer suggests it was designed for cars which already have the screen separated from the electronics in the way the D3 does, although there are pics of it in classics too, mounted in the manner you describe.
I see it as something which could occupy all the real estate in the D2's dash, which would make a welcome change from a 6" screen in an aftermarket unit which is then surrounded by bezels and wasted space. It may be that the single din control unit could sit behind it, or it may be able to be tucked away in the boot with the Bose amp depending on the cable length.
If it was available in the UK it would be worth finding one and measuring it
Edit - Just looking at Pioneer's UK site, they do actually have exactly that - a floating screen unit designed to go in a single-din dash aperture. That's different to the remote-electronics one which Johannes found though, which we don't appear to get. The floating screen units are 8" so would be an easier fit in the dash, but the screen does not appear to decouple so wouldn't work in this context.
Edit edit - it looks like the floating screen unit simply uses the single-din slot as a mounting bracket, and in fact has all the electronics in the screen unit itself. That definitely offers some new possibilities... More investigation needed
https://www.halfords.com/technology/...eo-212902.html