We had one fight where our halfling wizard took out like four minions using unarmed Opportunity Attacks.
Even if you have something in your hands, you could kick someone as an unarmed attack. I think, at least: in 3rd ed, it specified that any part of your body could be used for an unarmed attack. 4th ed is less clear on unarmed attacks.
You're not likely to actually hit with an unarmed attack (no proficiency bonus), especially since the people most likely to be unarmed (spellcasters and the like) are the worst at basic melee attacks anyway. But for flanking purposes you just have to threaten, which you can indeed do, as far as I can tell.