We spend AGES on this yesterday and discovered that it is possible! Essentially, someone has to "own" the conference. So in my example of the ad-hoc, rather than just hanging up, we have to put the conf on hold and then hang up. It uses one the call keys and conference on the phone, but the external parties can continue to chat. Once they hang up, the call key automatically clears. This is quite good because we could get back into the conference if we wanted to.
The other way we did it was to indeed use a meet me conf. We setup a conf access code and then called the conf assistant. From there, we were able to add callers to the conference (we have 5360 handsets so use the touch screen, not sure what the feature code is!) and dial them in. Once they were all added, we could hang up and leave them to it without utilizing a call key. The callers come straight into the conf, so don't need to worry about the access code etc.
We're nervous about setting up access codes and dishing them out to everyone as I've no doubt the cheeky customers we have would discover they could use our PBX as their free call conferencing gateway. I hope mitel one day set up a "Leaders PIN" facility so that the conference can't start until the leader has entered the room!
Thanks for your input on this...