Can I use Schedule 40 PVC to protect a grounding electrode conductor where exposed to physical damage?
After some more research, the answer appears to be no. Per section 352.10(F) of the 2014 NEC and its informational note (quoted below), only schedule 80 is allowed where PVC is exposed to physical damage.


  Below Quotation is from NFPA-70:2014; The 2014 National Electrical Code
  
  352.10 Uses Permitted.
  
  ...
  
  (F) Exposed. PVC conduit shall be permitted for exposed
  work. PVC conduit used exposed in areas of physical damage
  shall be identified for the use.
  
  Informational Note: PVC Conduit, Type Schedule 80, is identified for areas of physical damage.