It would certainly lessen the strength of the wall compared to a solid wall of the same thickness, but making an earthbag wall thick
enough to fit a duct would cause it to be significantly stronger than usual. So I wouldn't be too concerned about strength.
This idea would be giving equal heating to the interior and the exterior, however, effectively cutting the usefulness in half. I think you would be much better served by doing it on an internal wall. Alternatively, I would insulate the inside of your wall and add the earthbags with duct run inside that.