Any finish on the inside of a guitar seems to be frowned upon and simply not done. There's nothing stopping acoustic guitars having finish applied to the insides during construction, but it rarely happens, even though it would help make them less vulnerable to temperature and humidity changes.

Maybe a paint finish is too reflective and makes the guitars sound too harsh? Though on something with F-holes, you need only spray the area below the F-hole, and you could always spray a stain, which won't add any extra sound reflections.