rpinger I must admit your request seems a bit impractical to me ..
Encrypting a prop. protocol; that in itself if a great security feature. Not to sound like a troll but if layer 18 security is your concern for a wireless link, forget about it; in the world of networking there is an axiom which says anything wireless CAN be hacked by an avid and pissed off enough hacker.
Enuff with the negative talk tho, what id suggest to make it extremely hard for the would be snooper would be (as others mentioned) is to place encrypted tunnels within the nstreme link.