This is a diplomatic term.
Any American who was a diplomat would instantly know what it means.
Most Americans are not diplomats, and have little reason to pay any attention to diplomatic titles or duties.
I therefore think many people would never have heard of the term, and that the best you could expect from most of those who had heard the term before is what ewie said above: "it's some kind of diplomat, isn't it?"
It looks like have a higher regard than many for the knowledge of Americans regarding diplomatic relations. I figure a decent percentage are cognizant that a chargé runs a mission when the ambassador is out of the country. Perhaps recent events in North Africa have heightened this awareness.