The OED gives the earliest use of "agent" as someone who acts on behalf of a principal as 1593, and the earliest use of "agent" as someone who acts or exercises power (as opposed to being used or acted on) to 1600. So the legal usage predates what you're calling the "postmodern" usage by a whole seven years.
http://dictionary.oed.com/cgi/display/50004315?keytype=ref&ijkey=enP14Sc5LqXT6
-^^^^^^^ CB: Interesting. The legal concept contains both the acting on behalf of and the active , not passive element. A legal agent acts on behalf of the principle who is passive.
I might be wrong, but I take the postmodern usage of "agent" as equivalent to "subject" , self-determined and active. The legal "agent" is active , but not self-determined; active , but "used" in your terminology. The opposition is between self-determined and other-determined, or user and usee.