Yup, I think that's a good idea. It's true that, from QExpo to QExpo, I have the feeling that booths were filled later and later, sometimes at the very end of the event. If, when QExpo starts, more than half the booths are empty or almost empty, it really does not look good, because that's when a lot of people from the gaming community at large will be checking the site for the first time (and last if they are dissapointed with what they find). And I personally think it's better for the site to have no booth than empty booths.
So giving more time before is a very good idea. Maybe the moderators could also be a little tougher and "activate" booths as "visible" only when they consider the contents as sufficiently rich (and it wouldn't need to be a release, obviously, but at least some text contents and screenshots, for example). That might piss off some people, but then if they have really not much to offer, then I think it makes sense to delay their participation to QExpo. It's a very basic and logical rule, when you think about it. There could be a "list" of "possibly upcoming booths" but without any link to the actual (empty) booth. It might be a good a good incentive for participants to actually try to do/release something, too.
A possible side-effect is that some people might give up faster because, not having received any exposure, they feel they can just abandon whatever plan they had and noone would notice anyway. Once again, though, the idea would be to define what would be meant by "minimal" contents and make sure that those people with ideas and potential are encouraged to offer it so that their booth can be visible from the start of the event on.