Tod~
The more I read on this topic, the more confusion I find. Being an ex-government type, I like to go right to the language in the regs (and not rely on the synopses found in boating catalogs and NGO sites and the like).
re stern light: As I read the USGS regs, in addition to the port and starboard side lights (for a power vessel < 12 m) one needs: Part C, Rule 21, (c) "Sternlight" means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135 degrees and so fixed as to show the light 67.5 degrees from right aft on each side of the vessel. [Fortunately, they have a clear diagram right in the regs.]
Alternatively, one can use an "all-around" (360 degrees) white light - which is really intended as a mooring light.
My thinking is that a light on top of the OB satisfies the latter requirement even if I am standing at the helm - because I am ahead of the requisite 135 degree circle. In fact, most stern lights I see mounted on poles on the starboard quarter - on all kinds of boats, not just duckboats - are frequently eclipsed by tall engines (think Mercs) as well as windshields, spray dodgers, etc. The location atop the OB prevents such obstruction and certainly satisfies the "as nearly as practicable at the stern" requirement.
Does this square with your understanding ?
All the best,
SJS