Extension (flexible) cords cannot be run concealed; they cannot be inside the riser, no matter what you call the riser. Could you use a typical (ie NM) wiring method in the riser, then connect with a flexible cord? Maybe, but there are guidelines for it and it's really square-peg-round-holing it. If you want to call the riser an 'appliance', you have an unlisted appliance which is subject to the approval of the AHJ.
Best case scenario would be considering the riser as an 'appliance' and connecting the appliance with a flexible cord to the premises wiring. You would still need to wire the interior of the 'appliance' appropriately (ie not with flexible cord).
Worst case, the riser is a 'floor', is for all purposes permanent (you're not going to move it out of the way and vacuum under it), and must be wired accordingly.
400.7 Uses Permitted.
(A) Uses. Flexible cords and cables shall be used only for the following:
(2) Wiring of luminaires
(3) Connection of portable luminaires, portable and mobile signs, or appliances
(4) Elevator cables
(5) Wiring of cranes and hoists
(6) Connection of utilization equipment to facilitate frequent interchange
(7) Prevention of the transmission of noise or vibration
(8) Appliances where the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance and repair, and the appliance is intended or identified for flexible cord connection
(9) Connection of moving parts
(10) Where specifically permitted elsewhere in this Code
400.8 Uses Not Permitted. Unless specifically permitted in 400.7, flexible cords and cables shall not be used for the following:
(1) As a substitute for the fixed wiring of a structure
(2) Where run through holes in walls, structural ceilings, suspended ceilings, dropped ceilings, or floors
(3) Where run through doorways, windows, or similar openings
(4) Where attached to building surfaces
Exception to (4): Flexible cord and cable shall be permitted to be attached to building surfaces in accordance with the provisions of 368.56(B)
(5) Where concealed by walls, floors, or ceilings or located above suspended or dropped ceilings
(6) Where installed in raceways, except as otherwise permitted in this Code
(7) Where subject to physical damage
90.4 Enforcement. This Code is intended to be suitable
for mandatory application by governmental bodies that exercise
legal jurisdiction over electrical installations, including
signaling and communications systems, and for use by
insurance inspectors. The authority having jurisdiction for
enforcement of the Code has the responsibility for making
interpretations of the rules, for deciding on the approval of
and materials, and for granting the special permission
contemplated in a number of the rules.
110.2 Approval. The conductors and equipment
or permitted by this Code shall be acceptable only if