Ever flown with a pony...?
I have removed the valve from mine since that rule started, always covering with 2" electrical tape, each end folded back for easy removal. I have watched TSA agents remove it halfway and replace it, more commonly to just look at it on the X-ray or Catscan seeing that the valve is removed - with no interest. Returning, I seldom get to see agents do the inspections, but never had a problem.
I have thought about packing it inside a transparent bag with the end secured by a twisty, but that would also violate the letter of the rule - and be more likely to get left open I think. Above all I want the inside of my pony to stay clean.
I have
never heard of anyone losing a pony in checked bags.