The operative verb here (as correctly listed by the OP) is προγράφω (prographó) whose meaning is given by BDAG as:
- 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.
- 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".
The Cambridge Commentary offers this:
Paul represents his previous preaching of Christ as crucified to the Galatians figuratively as a writing, which he had previously written (προεγράφη) in their hearts (ἐν ὑμῖν). Comp. 2 Corinthians 3:2 f. In this view κατʼ ὀφθαλμούς is that trait of the figure, by which the personal oral instruction is characterized: Paul formerly wrote Christ before their eyes in their hearts, when he stood before them and preached the word of the cross, which through his preaching impressed itself on their hearts. By his vivid illustration he recalls the fact to his readers, who had just been so misled by a preaching altogether different (Galatians 1:6).
Ellicott is for forceful:
Evidently set forth.—This hardly brings out the full force of the metaphor, which is that of a picture or writing conspicuously and publicly exhibited.
MacLaren is similar:
‘Jesus Christ crucified has been conspicuously set forth before you,’ he says to these Galatians. Now, he is referring, of course, to his own work of preaching the Gospel to them at the beginning. And the vivid metaphor suggests very strikingly two things. We see in it the Apostle’s notion of what He had to do. His had been a very humble office, simply to hang up a proclamation. The one virtue of a proclamation is that it should be brief and plain. It must be authoritative, it must be urgent, it must be ‘writ large,’ it must be easily intelligible. And he that makes it public has nothing to do except to fasten it up, and make sure that it is legible. If I might venture into modern phraseology, what Paul means is that he was neither more nor less than a bill-sticker, that he went out with the placards and fastened them up.