I just realized a very interesting consequence of two theses that I'm sure many people hold. The first thesis is the Reformed version of infant baptism. I don't think the Catholic version has this result. Protestant paedobaptists believe that baptism does not save, nor remove original sin. It does indicate parents' trust that their child, being in the covenant community, will eventually develop personal faith and serve Christ as Lord. In other words, the content is pretty much what other parents express when they dedicate their children in hope of baptism when they express faith at an old enough age to be recognized as genuine. The biggest difference is that paedobaptists use the term 'baptism' and treat their children as part of the covenant itself rather than simply as benefiting from being part of the more general covenant community. As a credobaptist I disagree with this view, but it's quite common among the Reformed.
The second thesis is the main idea behind retroactive prayer. I've already argued for the possibility of legitimately retroactive prayer, so I won't do all that again. I'll simply say that if you accept that God has perfect knowledge of what's future to us (whether because he's outside time or some other reason), there's no reason for us not to pray about things that have already happened when we don't know the outcome. If God can foresee my prayer beforehand, then there's nothing to stop God from answering what I will later pray. The key idea here is foreknowledge, which all Reformed accept.
Now if you put these two things together, you get a very surprising result. The same things that justify these two views, when combined, will open up the possibility of baptism for the dead.