Non-metallic conduit support on horizontal surface in inaccessible space
You might want to check with the inspector first, but I'm going to say they'll allow it.  NEC allows PVC to be supported by passing through framing members, so I'm guessing they'd allow it to be supported where running over framing members (especially in inaccessible areas).

National Electrical Code 2014
Chapter 3 Wiring and Protection
Article 352 Rigid Polyvinyl Chloride Conduit: Type PVC
352.30 Securing and Supporting. PVC conduit shall be installed as a complete system as provided in 300.18 and shall be fastened so that movement from thermal expansion or contraction is permitted. PVC conduit shall be securely fastened and supported in accordance with 352.30(A) and (B).
(B) Supports. ...Horizontal runs of
PVC conduit supported by openings through framing members
at intervals not exceeding those in Table 352.30 and
securely fastened within 900 mm (3 ft) of termination
points shall be permitted.