For example, if I have a friend's public key that's associated with, say, email address `john@domain.com`, but I want to send him a message at `john@other.net`, is there a sensible way to do this? Is there a way to permanently associate `john@other.net` with the original public key?

I realize I could manually encrypt from the command-line (via the original email handle) and subsequently email that content, but I'm looking for something 'usable' that would properly integrate with my email client and other standard GPG/PGP tools.

I also realize it's possible to [add a secondary uid/email address to a key you own](https://superuser.com/questions/293184/one-gnupg-pgp-key-pair-two-emails), but neither is that what I'm looking for.