It's in the Pro repository now. I'll be making a new NGUI tutorial video this week, so I'll show it in action.
Fun fact: it was a feature I really wanted to add to uGUI, but I lacked the expertise/know-how, and others weren't sure how to do it either I guess, so it was cut. Last night (well technically still this morning for me since I've been up since yesterday)... but anyway... last "night" I had an idea, and sat down to do it. 12 hours later, voila.
I have to admit it's actually incredibly useful... I made sure that it's not limited to NGUI, and supports everything that has a preview in Unity and evaluates to a game object (read: prefab). I could adjust it to support more than that if I wanted -- audio clips for example, but we'll see.
As it stands right now, when you drag something onto the bar, it will prompt to you to create a prefab (unless it's already a prefab). It will create a preview for it, and from that point onward you can use it like any other prefab and then some -- drag it off that bar into the scene, drag it around to re-arrange the prefabs, remove, insert new ones in-between of others, etc.
The fact that it supports preview of NGUI widgets is a neat bonus. It will automatically search your project for prefabs named "Control -" and will add them to the toolbar on startup. This is what it looks like when imported into a default NGUI project:
