Tosfos (Gittin 13a, s.v. לא יתנו) write that
דאע"ג שמינה המגרש בחייו השליח לא חשיב להיות כמותו אחר מותו כאילו הוא עצמו קיים אלא חשיב גט לאחר מיתה
Even though the husband appointed his messenger before he died, we don't say that "a person's messenger is like him"* after he died, to consider it as if he (the husband) is still alive; rather, this is considered to be a case of "divorce after death" (which doesn't work)
Are there any rishonim who hold that there is such a thing as a "שלוחו של אדם כמותו לאחר מותו," a messenger-clone that works after death, as if the person would still be alive?
* A Talmudic principle, which means (according to most understandings of it) that the messenger of a person is exactly like the person himself. If this principle would apply posthumously, then this would mean that the dead man would be living vicariously through the messenger, and the messenger would be able to affect certain things that only that specific person could do. See the shlichus-agency tag wiki for more info.