Well, there was
RealID Toons Fix but it hasn't been updated in quite a while, and does not currently work, according to the last person I suggested it to. I glanced at the code, but it seems massively overcomplicated for what it's supposed to do, and I use separate Battle.net accounts so I don't need it anyway, so I didn't try to fix it.
The feature has been requested for my own multiboxing addon (
Hydra) a few times, and I'll probably get around to adding it at some point.