Jsabine- you have a fair point in that many long calendar rides do not start in places over-endowed with accommodation options. So, it might well be necessary for a prospective organiser like me to select a novel ride route (I could, say, have my riders out from Old Trafford and into unclassified country lanes within 30 mins).
Back to back 300s does sound a bit contrary to the concept (rather too similar to a 'traditional' 600km). So 2 x 200km or 3 x 200km seems, to me, right for riders who want a decent night's meal and sleep and to wake up the next morning feeling like human beings.
Having organised multiple conferences, training events and one wedding, my encounters with anyone who feels it is my responsibility to fix their accommodation have been scarce. If and when I do encounter such folk, I explain where responsibility lies. Are the circumstances in which this is likely to happen in a 'neutralised night stop' Audax any more likely than would be the case in a 'traditional' long event which starts early morning in a place where most riders would need to find a B&B or campground the night before (e.g BCM)?
Yes, a YACF group ride, is one way of testing the concept. This thread is another way testing the potential appeal. I think the question of how many love night riding (and can deal with the effects of this the next day) and how many would happily not bother with it is important.