Hmm, sounds possible .. lets see.
Event to watch : "PARTY_INVITE_REQUEST"
If Event is caught, send whisper to chosen named person with the information.
Or ...
If event is caught, send
http://www.wowwiki.com/API_SendAddonMessage which generates a "CHAT_MSG_ADDON" event which could then be trapped to display a message in the middle of the screen instead. This of course would work if you were using the same install or had the addon in both installations.
I'll leave it here for anyone that might want to do this if it doesn't already exist and I'll forget about it until the OP mentions that they still haven't found anything after a week or two.