I usually travel on a U.S. diplomatic passport and it's my understanding that USG travelers are not permitted to use the Diplomatic lane when returning to the U.S. Those lines are intended for non-U.S. diplomats arriving the U.S. Really, USG travelers aren't entitled to any special benefits when returning to the U.S. and there aren't any immunities and exemptions that I know of. When I enter other countries on my diplomatic passport, I do use diplomatic passport holder lanes when they're avaiable.