Change the "Private Message"
 

I want anything and everything to do with private messages to be owlery.

You have 2 new OWLs in the Owlery, etc.

Hacks? Templates?

MmmCOUGH LOOK over here

Phrases

Search in the search for phrases section of the ACP for private messages in the search and replace function and use whatever you want your replacement text to be in the bottom of the two boxes. Then repeat the process for private message.

Make sure you do it in that order, and do NOT search and replace for just private as that affects other non private message phrases and also the links to the private message system.