If you smash the barb down on the hook is it legal to use as a barbless hook?

Definitely, it's acceptable to mash the barb and use the hook while fishing in a "barbless only" water.  I have a two-sided tackle box.  On one side I keep all my barbless gear.  On the other is all the barbed equipment. When you mash the barb down, the best way to test is to run the hook through a piece of cloth (I use my hand drying towel).  If the hook snags when its withdrawn from the cloth, then you need to pinch the barb again.  If it comes out clean - you're ready to fish.