The scale of these conferences necessitates a strict badge rule. To get a badge, you need a registration. No badge = asked to leave. The security drones don't have the authority to issue badges on the spot and are programmed to kick out non-badge holders. Good for you that you would have cancelled 30 mins before the talk but it still would be a breach of contract most likely.
I took a +1 to an RSA talk without a problem. RSA is gigantic (it eats Moscone) and badge-controlled. I just said "I need this person here in order to do the talk".
The typical solution is to assign a person with the authority to let +1 guests in to the registration desk or at least make sure that such a person can be contacted when needed. This is fairly common in my experience regardless of the scale of the event so I'm not sure why you're trying to make it look like this was somehow unreasonable.
