What needs to be done to protect existing 12/2 cable in a stud wall being converted to a mass wall?
This is an unusual question but I think the code does address it and prohibit what you're doing:  


  334.12 Uses Not Permitted.
  
  (B) Types NM and NMS. Types NM and NMS cables
  shall not be used under the following conditions or in the
  following locations:
  
  (1) Where exposed to corrosive fumes or vapors
  
  (2) Where embedded in masonry, concrete, adobe, fill, or
  plaster
  
  (3) In a shallow chase in masonry, concrete, or adobe and
  covered with plaster, adobe, or similar finish
  
  (4) In wet or damp locations


More general answer - when you have an unusual setup it's best to err towards caution;  I'd say you need to do something other than just fill around that cable.