Yes, you would have to enable relay browsing for that to work, which would be really cool... Or maybe just a slider on the post editor that enables publishing to a specific address that is manually typed in, that overrides the relay list for that post. Then Amethyst user could create Zapbox notes and whatnot, if they want.

this is what it looks like in jumble (since jumble knows what relay(s) you were browsing from.) the toggle sets the nip70 flag.