[OSList] Half-day Open Space?

[Gray Miller]

I have a bit of a crisis as I am trying to negotiate an open space for a relatively large (500-700 attendee) conference that would like to incorporate Open Space.

Namely, I was just informed (via text message, before the meeting) that rather than having a full day, I will have about 6 hours (beginning at 11:30) to make the open space happen.

I already have some of the excellent words I've heard from Lisa Heft in mind for talking to the event manager. However, I'm curious if anyone else has had experiences with successful half-day Open Spaces (or other event models) and how that looked. I'd also be interested in hearing attempts that didn't work; my impulse initially is to say to the manager "perhaps open space is not the approach we should take here." 

However, part of that is being in unfamiliar territory; I've only done OS in multiple-day setups.

Any ideas or resources would be appreciated,

Gray