[lmilcin]

On most rubber dome keyboards there is enough space between rubbers to have diode there. You could also put diodes on the other side of PCB.

I am not sure about costs, though, I did not consider membrane keyboards for my research. Membranes are extremely cheap. Not just because of the membranes being cheap but also because there is no assembly (PCB is contacts, no assembly required for switches) and because there might not be need for double-sided PCB. In this case diodes could force double-sided PCB, vias, and maybe even double sided assembly.