Here is what Wallace says about it:
In Gal 4:19, Paul speaks of "my children, whom" (τέκνα μου οὕς), using the masculine relative pronoun to refer to the "children."
I also notice that in footnote 3 he gives some other examples:
See also, regarding τέκνον, Phlm 10 (παρακαλῶ σε περὶ τοῦ ἐμοῦ τέκνου, ὃν ἐγέννησα); 2 John 1 (τοῖς τέκνοις αὐτῆς, οὕς [in which the antecedent of the masculine οὕς is both a feminine singular and a neuter plural]). The word παιδίον is similar: cf. Mark 5:41 (κτρατήσας τῆς χειρὸς τοῦ παιδίου λέγει αὐτῇ), in which the feminine pronoun is bracketed by παιδίον and τὸ κοράσιον; Mark 9:24-26 (παιδίου... αὐτόν... νεκρός).
With Philemon 10, I would wonder if this isn't an issue of attraction to Ὀνήσιμον.
2 John 1 is covered by Smyth §1055, where it says that "[w]hen the persons are of different gender, the masculine prevails." Would this be subsumed under
ad sensum?
Mark 5:41 is certainly an instance, with the switch from τὸ παιδίον to αὐτῆς.
Mark 9:24-26 also offers an true example.
Does anyone else have anything to offer regarding the original verse that he referenced (Galatians 4:19)?
Jason A. Hare
Rehovot, Israel