1/2" plywood over the NM Cable would appear to an exhibitor a fine place to drive a nail to put up a flyer. [Linked Image]

I think to cite the physical damage aspect of the NM cable without also including the physical damage aspect of PVC40 conduit would be a half measure.

NEC352.12 - Rigid Nonmetallic Conduit, "Uses not Permitted"....
"(C) Physical Damage. Where subject to physical damage unless identified for such use."

NEC352.10, "Uses Permitted"
(F) Exposed. For exposed work where not subject to physical damage if identified for such use.

Unless it's PVC80 it's a violation, & it looks like that feeder has an even better chance of getting clobbered than the Romex.