That frosting was what we did (and where I even said it was done to diffuse the light) and then you said it's not transparent enough. Making it more transparent would make it less diffuseHm, the mat itself can spread the light I think, with the right "frosting" at least. But it indeed seems not be enough to go to the top of the butons without further tricks like the reflective material and more LEDs.
I once disassembled an very old Alcatel mobile phone to see how it works. It had a printed rubber keymat, pretty simple actualy. They used an additional white metal sheet underneeth which also acted as click button for each key. The LEDs went just through holes and lit up the rubber. With the white coat of the Pyra PCB the effect should be the same.