OK, so it's not exactly a Franiec design but it works. Tired of inadvertently pressing other buttons with my thumb, I cut out a thumb rest from a 3/16 inch thick self adhesive felt pad (intended to be used on furniture). The pad is thick enough to separate my thumb from the body of the G12. I actually contacted Richard Franiec and asked if he were making a G12 thumb rest but, at this point, he is not.
Although I had good intentions of shaping the pad to match the existing "support", the pad is not easily cut to curves. I cleaned the surface of the support with alcohol before applying the pad and it seems to be sticking very well.
So far, my thumb rest is working and seems to be an improvement. (Wonder if I can get it in black?)