Is there a way to simply splice in an additional length of 12/2 NM-B cable?
Tyco makes NM splices which are concealable and acceptable under NEC for use for rewiring in an existing building.

NM Cable Splices



NEC 2008


  334.40 Boxes and Fittings.
  
  (B) Devices of Insulating Material. Switch, outlet, and tap devices of insulating material shall be permitted to be used without
  boxes in exposed cable wiring and for rewiring in existing buildings
  where the cable is concealed and ﬁshed. Openings in such devices shall
  form a close ﬁt around the outer covering of the cable, and the device
  shall fully enclose the part of the cable from which any part of the
  covering has been removed. Where connections to conductors are by
  binding-screw terminals, there shall be available as many terminals as
  conductors.