I have been a Speedplay user since Drew introduced them to me in 2002 and I must say I really enjoy the float and the overall pedal system.  BUT, I have recently had an issue with a couple pares of cleats where the plastic 3 hole adapter plate becomes grooved from the "float" of my foot.  As the groove becomes more pronounced the pedal/cleat interface becomes a little sloppy.  Anyhow, I have no plans to change shoes from my 3-hole Cannondales (rebranded Diadora's) but was wondering if this would work.   The description sounds as if it is intended for attaching directly to the carbon soles of 4-hole shoes, but could I attach this between the cleat and adapter plate?  It seems simple enough but I wanted to check to see if anyone has used this set up. 

Thanks-Pete