Great stuff flying - we all do very little of it in absolute terms and then worry about all sorts of theoretical problems!
The AIC says the instructor is signing to state the pilot has completed a flight of one hour with an instructor. How can that possibly mean that that instructor has any liability? Remember it is not a competence check.
Indeed any instructor who refuses to sign is actually exposing themselves to action for theft/fraud - you pay for an hour to get the signature - the signature it then withheld on some competency grounds which are no supported in the offical documentation. Think about that interesting theoretical situation!
As with most of the JAA related stuff it is fodder for lawyers and not much use for the rest of us. Having said that I am one of those 'rogue private pilots' who never (usually) flys with an instructor. I have significantly more experience than many of the local instructors and fly in aircraft and locations they cannot. We simply operate in very different environments.
I too fell foul of the new rules and required to do the GFT route to re-validate - to be blunt it is no big deal. To anyone on a budget facing re-validation I would say forget all the currency stuff and just go and do the GFT. If you are reasonably experienced and think about your flying, you will easily pass.
Happy landings