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
--
"I had never understood why Socialism need imply the arraying
of oneself in a green curtain or a terra-cotta rug, or the
cultivation of flowing locks, blue shirts, and a peculiar cut
of clothes." -- Isabel Meredith, _A Girl Among the Anarchists_ Voyou Desoeuvre <http://blog.voyou.org/>