We definitely don't support this right now... and not sure if we ever will. For this to happen, the user would first have to accept your application, and then we'd have to check their friend count. I think it would be possible, but not sure if it would be easily useable to let the user know why they were denied and then, if they are denied, how to let them in when they finally meet the specified criteria.
Finally, this would also require some research into Facebook's terms of service. I don't think this would directly be discrimination, but there are things that Facebook doesn't allow you to do with 'their' users.
So, in short, not now, and likely not anytime in the future. If you need more information on possibilities, definitely let us know. Hope this helps, even if it's not what you're looking for.