How can I replace or repair termite damaged 2 x 6 tongue and groove sub flooring without altering the 1 x 6 hardwood floor on top of it?
i dont think it can be done.  the hardwood has to be nailed to the subfloor, and removing the subfloor from below would make it release the flooring.  on top of that, the stiffness of the floor is partially due to the connection between joists that the subfloor creates.  if you somehow removed the subfloor, putting it back would require small enough pieces to fit between the joists, which would substantially weaken the floor as the joists would not be connected any longer by the subfloor.

you could certainly reinforce it by adding sections of spruce plywood, and gluing them up against the subfloor from below.  use a good quality construction adhesive and then push it up from below with solid lumber blocking screwed laterally into the joists