I thought the standing position was well enough defined. We've had weeks of wrangling over what kneeling means.
I think a prescription for seated could be drafted without too much disharmony. We are part of the way there by defining the bag thickness etc already. How about, bum must be on a bag or on the ground, both feet must touch the ground but no demand for either sole to be flat or in any other plane, rifle to be supported by the body and no other external support or structure to be touched in any way by any part of the body or the kit.
What has been left out?