I just cloned the Zephyr repository and saw that the 6LoWPAN stack was the same as the one I am used to - e,g, the Contiki 6LoWPAN stack. How are you planning to develop it further? Are you going to sync it from Contiki or would you like Contiki developers to post pull-requests to fixes also to Zephyr?
Anyway - congratulations - it looks like an exciting project and I will be happy contribute to it!