Since the time canon was formed, when did the teaching that salvation can still be obtained by people after their physical death first appear in Christianity?
As far as I can tell, no one knows for certain where the doctrine of a second chance after death began (sometimes called "postmortem evangelism"). Catholicism invented the first major variety of this doctrine with it's concept of purgatory. Since then, many denominations, such as the Mormons, have developed different varieties of this concept. As with most false teaching, this is a product of misinterpreted Scriptures and man-made creeds. One of the most vital rules of interpretation is that ambiguous or difficult passages should be interpreted in light of plainer passages.
Here are the two Scriptures most often abused to support salvation for the unfaithful dead:
I Peter 3:18-20 & I Corinthians 15:29
In light of Hebrews 9:28, which plainly shows that man dies and then he is judged, as well as Luke 16:19-26, which plainly shows that the deceased righteous and wicked are separated by an impassible gulf, we can determine what these more complex passages illustrate. We can instantly flag any interpretation which conflicts with these as false teaching.
I Corinthians 15:29 is simple - Paul, in support of the resurrection, used the ungodly as an example. The point made here is, "Not even the ungodly deny the resurrection, or else they wouldn't baptize on behalf of the dead." Interestingly, Paul never stated that he, the other apostles, or the Christians at Corinth practiced such a thing. Note his use of pronouns.
I Peter 3:18-20 - Admittedly, this can be a very difficult passage. The context is the key to unlock the meaning. We should first review the flood account in Genesis 6-7. The elements here are salvation in Jesus, The Holy Spirit, preaching, lost souls, and God's patience. The people of Noah's day could have been saved by obeying the message that was preached to them. Jesus, being the source of salvation, issued a call to repentance to them in the Spirit through Noah ("a herald of righteousness") just as he did later through the prophets and the apostles. For 120 years, God was patient, but they rejected Noah's preaching, thus perishing in the flood in a lost state. At death, lost souls are cast into a "pit" or "prison" and reserved for judgment (II Peter 2:4-9). The NASB clarifies the tense of I Peter 3:19.
He went and made proclamation to the spirits now in prison
Jesus did not preach to the spirits after they were dead and imprisoned, but while they were alive and there was still hope, he preached to them through Noah. They refused, and are thus doomed along with the rich man in torment.
The idea shows up very early on, dating back to apostolic times. We see Paul talking about baptisms on behalf of the dead (1 Corinthians 15:29) as if it were a familiar, non-controversial practice, and in Peter tells how Christ went to preach to "the spirits in prison" while he was dead, (1 Peter 3:18-20) and saying that they were sinners who had been disobedient in their lives. So it's safe to say that the concept has been around since the beginning of Christianity.