it sounds as though some of these LARPs are what a different crowd would simply call acting exercises or dramatic long-form improvisation, rather than some totally new and never-before-seen art form.

Indeed. I've been returning to some of my sadly dusty acting textbooks and it is apparent to me that the closer LARP moves to this "extreme" model, the closer it is to dramatic improv. This isn't a bad thing - I'm constantly proselytizing various improv techniques to LARPers - but, yeah, it's not new overall... Then again, what is? ;)