On Tue, Dec 24, 2013 at 8:52 AM, Jeremy Spilman <jeremy@taplink.co> wrote:

Are there any past instances of applications hijacking or interfacing with
the exiting p2p messages, or abusing 'getaddr' functionality? Are there
any guidelines on this, or should there be?

There was a BIP by Stefan Thomas for adding custom services to the protocol. Discovery would be helpful here too. If this was added, it wouldn't be intended for use in a hostile way though.

This one was the custom services BIP. It defines a change to the version message and also custom sub-commands.
https://github.com/bitcoin/bips/blob/master/bip-0036.mediawiki

This one discusses how network discovery should be handles.
https://en.bitcoin.it/wiki/User:Justmoon/BIP_Draft:_Custom_Service_Discovery