Port knocking is discussed elsewhere in the thread - so you can see the problem with that.
I hope to be able to reduce the risks of "untested" by seeking as much tried and tested material as possible hence the reference to NetServer::Generic which I presume IS tested. I might be able to build the protocol over another for example, but I left that idea out to give people a chance to suggest it ;) e.g. Telnet. Of course I may be being naive about that idea - I'm not a networking guru, so I didn't want to put that idea in people's minds too early.