This has been posted before, and somebody actually made a plugin for MSN Messenger, but unfortunately it was very buggy and discontinued. The fact that he made the plugin basically proves that it is possible to create such a thing, but as for it being a feature of Messenger Plus!, I'm honestly not sure. Seeing as Messenger Plus! Live has it's own polygamy feature, i'd say the only issues would be finding a nice looking user interface and actually having the time to code such a thing from scratch.
Always a great idea though.
