Here is a way I belive that will let you advertise for Brickimedia without getting banned. Simply copy this text to your userpage and talkpage, put in your BM userpage/talkpage link and there you go. This is not really advertising per se, and I don't think Wikia will ban for it since you're simply telling a user who wants to contact you where to go.
<div style="align: left; padding: 1em; border:; background-color:purple; color:white"> <div style="align: left; padding: 1em; border:; background-color:black; color:red"> <big><big>THIS USER IS INACTIVE AND UNLIKELY TO RESPOND TO MESSAGES.</big></big> REASON FOR INACTIVITY: MOVED TO BRICKIMEDIA. IF YOU WISH TO CONTACT THIS USER, PLEASE GO [http://en.brickimedia.org/wiki/User:youruserpage HERE]. </div></div>
Forums - How to advertise for Brickimedia without getting banned