Well, I made it out alive!
I have to say that their reaction was remarkably positive to my (metaphorically) slapping them in the face! They did challenge some of my assertions (which is the least I expected) but the overall feedback I received was amazingly positive. In fact I received invitations to do more of the same from some of the participants.
The presentations I gave were both difficult to prepare and difficult to deliver because, normally when I am presenting, I am talking up some technology or process. This was the first time I was asked to deliver a negative message in a presentation. Still, now I know I can do that (and do it well, if the reaction of the participants is anything to go by).
The difficulty was, of course, tempered by the location!