Depending on how you want to set it up, just use externnotify= and a script that touches the msg<num>.txt file in the user's vm directory on the other boxes. We're using ssh, you may choose to use a different method. It's an immediate MWI notification, and seems to work well.
Find details at: http://asterisk.mdaniel.net/?p=14
Server to server MWI over IAX: MWI over IAX