The operative verb here (as correctly listed by the OP) is προγράφω (prographó) whose meaning is given by BDAG as:
1. to write in advance or before, write before(hand)
 - (a) in the same document in which the word is found, eg, Eph 3:3.
 - (b) what is written before is found in an older document (by another author as well), eg, Rom 15:4, Jude 4.
2. to set forth for public notice, show forth/portray publicly, proclaim or placard in public, Gal 3:1.

As usual, BDAG's definitions are based solidly on the voluminous Koine Greek literature of the first century; evidence for these meanings is listed in each word entry - see BDAG for details and its sources in such literature under the entry προγράφω.

Thus, the translation of προγράφω (prographó) as "publicly portrayed", or better, "placarded publicly" (and hence written beforehand!) is entirely consistent with the range of meaning of this verb.   Paul may also be hinting here of the placard ("titulus") affixed above Jesus on the cross saying, "Jesus of Nazareth, King of the Jews".