Hey, RDRickster. Getting hit from behind is a risk we all take, and one of which there is little we can do. As you say, we don't have eyes in the back of our head, and even if we did there's too much aircraft structure in the way for them to do any good. Bottom line: you just cannot look around too much, and sometimes that might not even be good enough.
Most helicopters are among the slowest craft in the air, and this puts us at special risk when being overtaken by seagulls, ultralights, hot-air balloons and the occasional errant fixed-wing. I vividly recall going up the Hudson River in a 206 one day and nearly being run over by a Cessna twin of some sort. Our ship had only the requisite single white anti-collision light on the top of the vertical fin (before the FAA, in a fit of sheer stupid madness mandated that they all be changed back to red). I subsequently felt quite outraged when my Chief Pilot rebuffed my request for better additional strobe lighting facing rearward, something I've come to believe is a necessity for helicopters. Where I live now, the slowest U.S. Navy trainers all have those neat combination tail position light/strobes, evidently to protect the little T-34's from being run over by F-18's (or each other).
So I sympathize with your anger at your near-hit.
Question though. Is there some sort of CTAF that the other aircraft should have been monitoring? Your post makes it sound like there was.