I've got one of Rich's membranes in my 800, I tested it a couple of weeks ago, i've been snowed under with work since, but here's my finding's:
The top row doesn't respond.
The second row doesn't respond up until the 'U' key, the rest of the keys on that row are fine.
The third row keys are fine to 'H', 'J' key onwards doesn't work and i'm not sure about CTRL.
With the final row, only keys 'N' onwards work
The space bar is fine.
I think it's just down to contacts between the keyboard and mylar and over this weekend i'm going to try some foam, paper etc and see what works best