I already do something like this regularly, but since there's no existing shorthand, it generally means adding something like, "I'm sure you must be busy, so if you literally want to just send a one or two word reply, that's no problem! Cheers." If there were a universally-known way to say that in 4 characters, it would make things a bit easier when both parties are busy. That said, you could also just use a macro to add the above to an email.
The closest parallel to the OP's suggestion that comes to mind is adding (nt) to the subject line of an email if there is no text, saving the reader the time of opening the body of the email. (Although this is made obsolete by most modern email programs, which show previews.)