While I'm hoping this is a hyperbolic suggestion, it's almost certainly a bad idea to use the clipboard for this sort of thing, because none of that is sandboxe

While I'm hoping this is a hyperbolic suggestion, it's almost certainly a bad idea to use the clipboard for this sort of thing, because none of that is sandboxed properly, and creating a special format that says, "I'M A PASSWORD" sounds to me like designing an API for malware authors, frankly. At the moment, it seems like copy-paste in passwords is a nice intermediate step between the "password you can remember" era and an era where we have secure keyring managers. In the end, you can imagine that you'd want them stored not on the clipboard (where anything - including Javascript running on a site - can get them easily), but in a secured area of memory, and entered on demand on trusted sites.