Just one thought, Nick:
One of the disadvantages of a tail rotor is the need for a long driveshaft, and additional gearboxes. Indeed this was my main concern with Dave's side-by-side concept layouts - i know how overlooked driveshaft installations often are. Albeit treatable as an engine failure, a tail rotor failure is not a nice event. Even twin engine aircraft still have one TR driveshaft, albeit suitably unstressed.
There must be some advantage in having the driving fan close to the main powertrain? This way "TR" only failure is less likely, since only modes losing MR drive would affect it. The coanda effect boom is a non-starter for me, since blending stabiliser into boom aerofoil makes more sense - and even this doesn't work with crosswinds or tail winds.
So howabout a mixer nozzle, for ItchyRivers to consider? It would use the Notar nozzle concept, but the coanda effect this time would be used to ingest as much surrounding air as possible. Possibly installed as if it was a fenestron. I doubt it would achieve tail rotor efficiency, but it might have applications for mid-size machines...
Mart