Shawn,
I don't recall anything specifically stated in the FARs about having a visual reference to remain VFR. The closest thing I could find is 91.155 that states that in Class G "a helicopter may be operated clear of clouds if operated at a speed that allows the pilot adequate opportunity to see any air traffic or obstruction in time to avoid a collision." No statement about ground reference, but one might interpret the ground as an "obstruction."
In all of the helicopters that I have flown (Bells, Robinsons, and Enstroms), all of which are VFR only, the Flight Manual states something to the effect that for night VFR flights visual reference must be maintained with the ground by means of adequate ground or celestial lighting, but it does not place that limitation on day VFR flights.
Interesting topic, I look forward to finding out what you discover.
Cheers,
Steven