First, if you read this link
http://www.wowinterface.com/forums/s...ad.php?t=21004 you'll see that the feature you are asking about is already planned. In addition, if you go to nUI's download page at
http://www.wowinterface.com/downloads/index.php?cid=125 and check the "nUI Enhancements" section, you'll find that there's already an optional plugin there that does what you're asking for.