Looking at the UL white book, I see no info either way. Looking at the wiremold installation instructions I see no direction either, although the illustrations show base pieces being mounted both horizontally and vertically (but not the receptacles - they are always horizontal in the pictures)
I would venture my opinion that mounting wiremold 4000 receptacles face-up on the ground should be permitted in some instances, but may not be such a good idea in all locations. The NEC requires all installations to be "acceptable to the AHJ" and to be "suitable for the conditions of use".(110.2 & .3)
It would be my recommendation, then, that prior to installation, you contact your AHJ for their acceptance of the plan.
