I haven't figured out all the details for how the "membrane" keyboard will work , but I do know that I will be needing the artwork that rests on top of the switches, 7-segment displays, and LEDs. So I spent a couple of days re-learning Inkscape (since I don't use it enough to be truly proficient), and produced this:  

I started with a scan of the originals keyboard:

which I brought in to Inkscape at a 1:1 scale as the bottom most layer and basically traced the various elements using Inscapes vast array of drawing tools on "higher" layers. I was also lucky enough to find a font, "Christ 100", that was a pretty close match to the originals which saved a lot of time.  Not so lucky with the A-H 1-8 button characters which had to be carefully traced and rendered as lines and Bezier curves.

As a quick check of the fit:

• I printed the artwork on a transparency (to avoid ink bleeding into paper),
• laminated the transparency backed by white cardstock using 3 mil film,
• cut the keypad membrane artwork to size,
• and attached the result to the inside of the CHESSmate top case with two sided tape.

A perfect fit.