How about a option as part of Plus! Preferences in which you would be able to set a display name ex) with color / format codes. Then have a dialog where you can select all of your contacts whom you know are using Plus!
For your Contacts which you have not selected to see your Plus! Friendly name, they will see whatever you have set in WLM's default Display Name field.
So..
1. Selected Contacts in the Plus! Prefernces will use the Plus! Display Name Field
2. Unselected will use WLM default Display Name Field.
I donno, its a stretch... i have no idea how WLM & plus! can work together
