The answer is there's no definitive answer because the question hasn't been ruled on in court yet, AFAIK. By the wording, no, you could not be convicted unless your shotgun held more than two rounds in the magazine and one in the chamber. Clearly, the law did not contemplate floating a round when it was written. It's an easy oversight to fix up in legislation, if it came to that.
So, by the letter of the law is it legal? Yes. Is it reasonable that some CO may not see it that way and let the court decide? Yes. Could you have to spend many thousands of dollars and many months winning the point? Yes. If not illegal, is it against the intent and the spirit of the migratory bird regs? Yes. Is it unsporting to bend game laws and exploit technical loopholes to gain an unfair advantage? Yes. Is it far too much trouble anyway, and likely much much less fun in a wet blind with freezing fingers than it is on the range? I would strongly suspect that's a yes too.
Pretty much sums up the topic to me.
What was the first shotgun capable of reliably floating a round anyway?