I don't think such a mod does exist. We would have to make it.

First, you have to add the custom buff to buffs.xml via editing the config file or doing a modlet for it.

And to clarify on how I see this flow happening technically on the server:

• Player goes south beyond PvP border.
• Server manager detects player location and sets PvP mode (already happening today automatically).
• Server manager uses the "buffplayer" command to apply the custom buff.
• The buff will have a very long or infinite duration (I'm not sure if it's possible to have infinite duration buffs?), and is re-applied by server manager as soon as it wears off (if infinite duration isn't possible).
• If player goes North into PvE again, the buff is removed immediately with the "debuffplayer" command.