Post is one of English's newer prepositions. The OED has it from 1965,
"1965 Listener 16 Sept. 432/3 Der Ferne Klang is post-Wagnerian, and post just about everything else that was happening at the turn of the century.
1974Daily Tel. 7 Jan. 13/3 Now, post the increase [in the price of oil],..future gold price prospects far outweigh individual share fundamentals."
About 15 minutes of quick and dirty searching through Google books wasn't able to turn up anything earlier. I also had a look through the Time corpus, the BNC, and the BYU Corpus of American English, but found that the part-of-speech tagging was uncharacteristically wrong when it came to post. There are thousands of hits for the preposition post, but I found not a single one that was actually a preposition.