We are planning to add in PK in a future version of ASL. Heres the original set of scripts we published back in 2004 that illustrates one method:

http://www.gotroot.com/blogpost7-Portknocking-in-BASH

We're looking at methods that don't require a listener, which means less chance of the PK server causing the box to get owned - so we like the idea of using the firewall system to do the heavy lifting for us. We don't pretend to have a monoply on great ideas, so if you have a thought on the matter please post them here.