Proxy baptism for the dead?

Otherwise, what do people mean by being baptized on behalf of the dead? If the dead are not raised at all, why are people baptized on their behalf? (1 Cor 15:29). 

This is a famously cryptic statement. I'll take a stab at it. There may be two semantic ambiguities in the text which commentators typically overlook. 

i) Conventional translations render the preposition in this passage as substitutionary: "for, on behalf of". Although that's certainly a legitimate meaning of hyper, the preposition has other senses in Greek. For instance, it can also mean "because of, in view of" (e.g. Louw & Nida, 89:28). 

ii) Suppose we plug that into the text. What might it mean to say Christians are baptized with a view to the dead? Well, in context, Paul is defending the afterlife (i.e. resurrection of Christ, resurrection of the just). Perhaps he means baptism is a witness to the hope of life beyond the grave. The resurrection of the just. Death is not the end, but a portal to a better life on the other side of the grave–for those who die in Christ. 

iii) Then there's the question of how best to render baptizo. Conventional translations render that sacramentally. But at this early stage in Christian Greek usage, were the noun and verb already technical terms for the rite of initiation? It doesn't mean "baptize" in secular Greek. How long did it take for the word to acquire the specialized sense of "baptize/baptism" in Christian Greek? I don't think we can simply assume that's the default meaning of the word at the time of writing (c. 55 AD). 

Suppose, instead, it's a metaphor for ritual purification. And suppose we combine that with the alternative rendering of hyper. What might it mean to say Christians were washed with a view to the dead? 

Here's one possibility: in the ancient world you often had a cult of the dead. Fear of the dead. Fear of ghosts. A felt need to placate ancestral spirits. Provide ritual libations at the grave. Magical relics. Necromancy. And so on and so forth. 

Perhaps 1 Cor 15:29 is a figurative way of saying Christians have been liberated from such anxieties and superstitions. 

Admittedly, both these interpretations go beyond what the text says or implies, but any interpretation of this passage must read between the lines because we've lost the background information which Paul's original audience had at their fingertips.