Can two ground rods be bonded via a PVC conduit above ground?
I can find nowhere in the NEC that says that you must bury inter-electrode bonding jumpers.  In fact, 250.64(B) explicitly allows for running grounding electrode conductors (such as inter-electrode bonding jumpers) along construction or otherwise aboveground when suitably protected against physical damage, etc.:


  Securing and Protection Against Physical Damage.
  Where exposed, a grounding electrode conductor or its enclosure shall be 
  securely fastened to the surface on which it is carried. Grounding electrode 
  conductors shall be permitted to be installed on or through framing members. A 
  4 AWG or larger copper or aluminum grounding electrode conductor shall be 
  protected if exposed to physical damage. A 6 AWG grounding electrode conductor 
  that is free from exposure to physical damage shall be permitted to be run 
  along the surface of the building construction without metal covering or 
  protection if it is securely fastened to the construction; otherwise, it shall 
  be protected in rigid metal conduit (RMC), intermediate metal conduit (IMC), 
  rigid polyvinyl chloride conduit (PVC), reinforced thermosetting resin conduit 
  (RTRC), electrical metallic tubing (EMT), or cable armor. Grounding electrode 
  conductors smaller than 6 AWG shall be protected in RMC, IMC, PVC, RTRC, EMT, 
  or cable armor. Grounding electrode conductors and grounding electrode bonding 
  jumpers shall not be required to comply with 300.5.


Of course, you'll have to double-check with your local inspectors, though -- they may have some local requirement for burial of bonding jumpers.