Could get a piston upper and turn the gas system off when you want to use it in Straight Pull Mode.
I doubt that a system that's "changeable in the field" would pass inspection. With the gas port blocked and the gas tube missing, there's no question.
A bunny cop checked me one time for a shotgun plug. He shook the empty pump gun and commented "No plug" since many/most wooden plugs clunk in the tube. I told him it was plugged so he wanted the barrel pulled-no plug fell out or was visible. Aha, he's going to write me a ticket. I bet him $100 he could only put 2 shells in the mag so he tried--no go. That gun had a custom made tubular plug fitted to the inside of the magazine tube. Regulations did not specify what TYPE of plug, just that no more than 2 shells could be in the mag.