Dave, wouldn't putting a ring around the tail rotor improve it's efficiency for less mass...

Bug, maybe you should quote some numbers here for comparison against an equivalent conventional rotor system. How much does ring-rotor weigh? How much thrust does the ring rotor produce? From 1g hover, how long before loss of driving torque becomes critical?

This will set the discussion up with a good basis for performance benefit comparisons... ;)

Ring rotor
 

The interference drag at blade tip/ ring intersection would be a big problem to address, I think.

This might prevent autorotation, as happens with tip jet drag.