Graeme,
Aren't there really two issues at place with using a clamp not specifically for Campagnolo?
The first is where the der will be positioned relative to the seat tube as you state but isn't the radius of the interface between the der and the mount also slightly different? I am working from memory here so I may be wrong on the second point.
Also, I know you could take a 10 spd der and remove the clamp and convert it between sizes or into a braze on (dont remember the part numbers) for Record and Chorus (IIRC). Is this still possible with 11spd? Looking briefly at mt 11spd SR mech it doesnt appear so.