Doesn't mIRC have this already?
Actually, it's called Active Mode, since DCC already is passive, and I can send to mIRC users using my client in Active Mode, anyway. It's possible that it requires the receiver to use /dccserver.
If you can get the receiver to use /dccserver, use /dcc send <ip> instead of /dcc send <nick> to signal that you're sending to someone's dcc server.