Regarding a later Siman: The Chavos Da’as Biurim 3, Pri Megadim SD 1, Chasma Sofer Bava Metzia 91a, Gilyon Maharsha, Zivchei Tzedek, Yad Yehuda, Ben Ish Chai, Badei HaShulchan and Artscroll all write that the Siman must be from the time of baking, and I didn’t find anyone who argues. Are you convinced now? Even the Chochmas Adam who mentions making asiman later explictly limits it to an inadvertant leakage of shmaltz and not an intentional addition.
Also, see the Kreisi u’Pleisi in the name of his grandfather who mentions dividing the dough into small portions among numerous people subsequent to the baking, and does not propose making a Siman. Apparently it is too late for a Siman, as the Issur has already descended upon the dough.