Following on from the discussion in the live chat today - thanks again Josh - the method I'd most like to try for the "friendly" dog owners who lag 50m behind their off leash dog who is incoming to my on-leash dogs, is the "Sure, I'll have your dog, see you later!" method.  

I always carry at least one spare leash with me on walks, usually a slip lead, on the chance that we meet a genuinely lost dog who needs help.  There've been a few times I've been tempted to pop it over the head of the friendly-but-pain-in-the-ass dog who's a far-off satellite of their owner, and then take off with the whole gang at a jog.  Potential for provoking human-human aggression would be high but geez it's tempting sometimes.  

I did once encourage a dog to come along with the gang and I off leash, and I did an abrupt 180 too, so we were rapidly increasing the distance between the owner and the dog.  It's amazing how quickly the owner made attempts to get the dog back once it was disappearing over the horizon.  

Segue to the topic of bicycling with dog/s running alongside... the whole bike + dog scenario does tend to trigger prey/chase responses in other dogs, so I've always got an eagle eye out for other dogs if I'm cruising along with dogs, and will tend to stop or go Very Slowly on occasions to reduce the arousal level in other dogs, especially if I am coming up behind a dog and their handler.  I usually call out to prepare the handler for the unexpected stimulus their dog's about to encounter.

All of those are good ideas. But I have to agree, that the potential for conflict is pretty high with the other dog owners if we take off with their dogs. Even if it would be a lesson well learned!