"The difficulty would be to find locations where people won't be used to hearing buskers, and at the same time succeeding in not being thrown out."
Crow Sister has it in a nutshell. When 27 musicians (plus some students) agreed to join in on April 17th, 2010, (Celtic Guerrillas Vancouver), that was the specific goal: do something unusual, but minimize the risk of wasting everyone's time by having the performance terminated.
Walking that fine line also calls for well-informed and disciplined participants, with a respected leader to follow.
What may look like casual videotaping probably involves many hours of editing in order to make a video presentable.

Just a point: "Commando Trad" refers to the Montreal group, and in my opinion, should not be used to describe the Liverpool activity.