I am proposing an unusual unusual understanding for this verse. Thistleton (NIGTC, p 1240) gives a summary of views on its meaning
It should be noted that BDAG gives 3 basic definitions for ὑπέρ:Verse 29 is a notoriously difficult crux: the most “hotly disputed” in the epistle (Conzelmann); “it is not clear precisely what this practice was” (Dale Martin); “everything must be understood as tentative” (Fee); a variety of understandings emerge “given the enigmatic nature of the practice” (Collins). By 1887 Godet had counted “about thirty explanations” for baptized for the dead,” while B. M. Foschini and R. Schnackenburg allude to “more than forty.” Wolff’s commentary includes seventeen subcategories with seven issue-centered general approaches.157 A vast literature stretches from the second century to the present day. Mathis Rissi devoted an entire book to this one verse, categorizing a mass of views on the history of interpretation under four main groups, with variations in each group. (a) One category adds σωμάτων to ὑπὲρ τῶν νεκρῶν, and identifies the dead with those who are being baptized. (b) A second view understands baptism as the suffering and death of martyrdom. (c) A third interprets baptism broadly as washing (where the Hebrew but not the Greek may use a common word). (d) The fourth understands this as vicarious baptism on behalf of people who are dead. Rissi rejects the “sacramentalism” often implied in this.
1. a marker indicating that an activity or event is in some entity’s interest, for, in behalf of, for the sake of someone/someth
2. marker of the moving cause or reason, because of, for the sake of, for
3. marker of general content, whether of a discourse or mental activity, about, concerning
I would suggest that what we have here is a classical use of ὑπέρ as given in LSJ: "of Place, over"
What we would have then is an early Christian practice of burying the dead under the floor of the church with the baptismal font being on top of the graves of the departed. Something of this is indicated by Re 6.9 Καὶ ὅτε ἤνοιξεν τὴν πέμπτην σφραγῖδα, εἶδον ὑποκάτω τοῦ θυσιαστηρίου τὰς ψυχὰς τῶν ἐσφαγμένων διὰ τὸν λόγον τοῦ θεοῦ καὶ διὰ τὴν μαρτυρίαν ἣν εἶχον.
This would be in keeping with Paul's discussion of the concept of resurrection in the context.