Far as I understand, there isn't a way to do that. Federated usernames work essentially like an email address because of the decentralized nature of the service. If I type sparky@lemmy.federate.cc it won't send you a notification, but it will link your profile.
At least I think that's how it works. Who wants to jump in and point out all the ways I'm wrong?
Not saying you are wrong, since it is currently imposible to do here, but I don't think the federated nature of usernames has anything yo do with if they receive a notification or bot since over at mastodon you can tag people properly and they are notified.