I've searched for information on this, but cannot find it, is this not something you can do yet with Kobold2D/Cocos2D? or are there options I've missed?
You can make your game using the Cocos2d framework and GameKit framework combined, there should not be any issue with that.
What you need to do is create the user interface using Cocos2d framework and then handle the turn based events using the classes provided by game kit framework.
If you are creating a card game (like uno) in Cocos2d, then you will be using the
CCSprites, etc. to display the cards and other components on the screen and for handling the turn based events like when a players turn comes to play a card from the pile in his hand you will have to use GameKit classes to send and receive the data from each player in the session.