Without the OED to consult, I have to fall back on smaller references, but the medical use of the word "plaster" is traceable in English back to O.E. "plaster", and before that to Low Latin "plastrum" and Latin "emplastrum" [A. Cornelius Ceisus, physician; c. 10 AD ?]  So it's probably been used in that sense ever since it was adopted into the English language, and -including Latin- a couple of thousand years, at any rate.  "Emplastron" ["emplaston"] was also used in Greek, but I don't know whether of medicine, construction or both.  (Refs. in this case, because to hand, Chambers 20th Century Dictionary and Smith's Smaller English-Latin Dictionary.

Malcolm