Steve_

I can't recall the statute off hand but I think it's more about don't misrepresent yourself. If you show up at a POE with a British passport and say you've lost your US one, and you make it absolutely plain to CBP that you've lost your US passport and only have your British one and you're a US citizen, they will probably send you to secondary and look you up to make sure you are. It's illegal to misrepresent yourself, not necessarily what ID you present, iirc.

I know they make a big song and dance about you must have a passport, trusted traveller card, blah blah blah but they can't deny entry to a US citizen. All it means is they check a variety of databases to confirm your identity. Memorize your alien registration number (especially if you're an LPR, in case you lose your card). CBP also have access to the DMV database, at least the one where the POE is located.

It's not like that Ben Stiller movie where he loses his passport and has to hire smugglers to get him back into the US.