Do all horizontal-surface mount outlets need to be "floor receptacles?"
You are correct in assuming that your ledge is not a floor. The NEC would define it as a work surface or a countertop. That would lead you to NEC 406.5 (E) &amp; (F) which would tell you that the device installation must be GFCI protected and listed for countertop and work surfaces. Most of the ones that I have seen are the pop-up type. A standard flush mount wall receptacle will not work. 

FYI. My other concern would be whether or not this 6" ledge top is a 2x6 plate on the top of a frame. Not that it matters code wise but cutting and mounting into it may pose to be a problem. You might want to confirm how this wall was constructed and plan it out before proceeding to cut holes in it.

Good Luck