>however attempting to create an account would disclose that fact by rejecting an account creation attempt with an existing email, unless they use emails purely as communication channels

They can tell the user to await an e-mail from them with the confirmation link. Then if the e-mail address is already in use, send an e-mail saying, "somebody, probably you, tried to register as <new-username> on <site> but we have you down as <old-username> already". Otherwise, send a normal confirmation link.