Hi Sphere, another resource you may have overlooked is your local sign shop.
They have computer graphic mylar tape cutters that can make any design you can think up. Prospective manufacturers are aware of the cost of custom parts like the keyboard you want, but they are kind of short sighted as to your vision of a product. You can make the simulated membrane keypad and present design of production model along with estimates of setup costs, UNLESS, Unless you intend to be the manufacturer too. Me, I would be happy with my 3 percent or whatever the market is now and move quickly to the next Million seller, and leave the headache of marketing and manufacturing to them,