Does Matthew 27:47 support praying to the saints?

Remarkably, some Roman Catholics and Eastern Orthodox cite the passage to that end. Let's think about the passage from a few different angles.

Does Matthew express any support for praying to Elijah? No. He just reports what some bystanders at the cross said, without giving any indication that he considers praying to Elijah acceptable.

Do those bystanders seem to think praying to Elijah is an acceptable practice? We can't tell either way from this passage or its parallel in Mark (15:34-36). People often suggested that Jesus did things they disapproved of (Matthew 11:19, 12:24). These bystanders at the cross are portrayed as ignorant, misinformed, and unbelieving. Given the general Roman and Jewish rejection of Jesus at the time and the general Roman and Jewish consensus that Jesus' being crucified was an indication of the gods' or God's disapproval of him, these bystanders could easily be attributing something to Jesus that they considered an unacceptable practice or something they were agnostic about. I see no reason to conclude that it's probable that these bystanders approved of praying to Elijah.

Even if they did, what if they were Roman soldiers (suggested by their not understanding what Jesus said, their being allowed to go up to the cross and offer Jesus a drink, and the parallel between Matthew 27:48 and Luke 23:36)? A Roman belief in praying to somebody no longer on earth wouldn't prove that most Jews at the time accepted the practice, much less that Christians accepted it. If the bystanders were Jews, we know that ancient Jews were often guilty of disobeying God in various ways, such as in the idolatry they're frequently criticized for in the Old Testament and the rejection of Jesus they were guilty of in the passage in question (if the bystanders were Jews). In fact, attempts to contact the dead (or people in the afterlife who hadn't died, like Elijah) were common in the Old Testament era, and those attempts are repeatedly condemned there. Furthermore, we know from ancient extrabiblical Jewish literature that some Jews in antiquity not only believed in praying to the deceased and angels, but also believed in praying to other entities to whom Catholics and Orthodox don't pray (e.g., praying to inanimate objects). And those Jews who prayed to entities other than God were accompanied by other Jews who opposed the practice. The view that we should pray only to God is earlier, is supported by the Bible, and is supported by the earliest extrabiblical Christians and many extrabiblical Jews. The fact that some ancient Jews practiced prayer to the dead and other forms of trying to contact the dead doesn't offer adequate support for the view of prayer advocated by Catholics and Orthodox. If the bystanders in Matthew 27 considered praying to Elijah acceptable, one way we can go about judging their assessment is by looking at how widespread such acceptance of the practice was elsewhere among the Biblical sources. Why is prayer to God supported so explicitly and frequently from Genesis onward, yet advocates of praying to the dead have to resort to such dubious evidence as the interpretation of Matthew 27:47 under consideration here to find alleged Biblical support for their position? If the bystanders in Matthew 27 supported praying to Elijah, the evidence suggests they were taking a problematic position that isn't supported by scripture and contradicts what the earliest Christians believed.

Then there's the fact that Elijah never died. While the acceptability of prayer to a human who didn't die, like Elijah, would address some of the objections to praying to the deceased, it wouldn't address all of them. The prohibitions of trying to contact the deceased (in scripture and among the earlier patristic sources) would remain an obstacle to praying to individuals who have died.

It could still be argued that the Matthew 27 passage at least shows that ancient Jews (if the bystanders were Jews) were familiar with the concept of praying to somebody like Elijah. But so what? People were showing interest in trying to contact individuals in the afterlife in one way or another from the time of the Pentateuch onward. The fact that ancient Jews were aware of the concept of praying to an individual like Elijah isn't in dispute. Citing Matthew 27 to show ancient Jewish awareness of the concept wouldn't accomplish much.