After reading all he posts and adding my experience, i would sum up by saying that turnbacks are a possibility given a whole lot of considerations:
a) it should only be considered in aircraft which have good L/D ratios and don't lose speed quickly at higher angles of attack (like most non aerobatic biplanes). It also helps if the aircraft has a high power to weight ratio and therefore gains height quickly after takeoff.
b) Given the above, the exercise must be practised at a safe height first.
c) Before takeoff, the parameters off minimum and maximum heights and therefore distance from the airfield must be decided.
d) The direction of turn must be considered depending on the crosswind (personally I favour turning towards the wind) and the airfield configuration.
e) The absolute necessity of maintaining safe airspeed in relation to the dynamic stall speed in a steep gliding turn.
f) Lastly, the first part of the turn must be flown without reference to the visual effects of slip and apparent increase in groundspeed.
Given all the above, anyone presented with a donk failure after takeoff and hasn't made all the correct decisions beforehand is better off going straight ahead.