Source: Philip Schaff, History of the Christian Church, Volume II: Ante-Nicene Christianity (Grand Rapids: William B. Eerdmans, 1910) 83.
"Origen went so far as to ascribe to the sufferings of the martyrs an atoning value for others, an efficacy like that of the sufferings of Christ, on the authority of such passages as 2 Cor. 12:15; Col. 1:24; 2 Tim. 4:6."
I read the same sort of idea in my Ignatius quiet time, but I have neither the time nor the energy to comb through Ignatius to find it. On the Internet, there are articles that associate "Ignatius" and "martyrdom" with "vicarious," so I know I didn't dream up the whole thing.
Let's look at the biblical citations that Origen appeals to:
II Corinthians 12:15: I will most gladly spend and be spent for you. If I love you more, am I to be loved less? (NRSV)
Colossians 1:24: I am now rejoicing in my sufferings for your sake, and in my flesh I am completing what is lacking in Christ's afflictions for the sake of his body, that is, the church.
II Timothy 4:6: As for me, I am already being poured out as a libation, and the time of my departure has come.
I don't see anything here about Paul making a vicarious atonement for his fellow believers. One can argue that Paul suffered for the sake of the church in the sense that he strove to feed it with the Gospel, putting his life and well-being on the line if need be. He would travel to edify the church, even if that entailed ship-wreaks and persecution. And his role as a libation may simply mean that he's consecrated for God's service, not that he's making atonement for the church.
At the same time, I'm reluctant to rule out a connection between martyrdom and atonement. Both Paul and Ignatius act as if their suffering is necessary. In their eyes, it's not a mere inconvenience that the world thrusts upon them, as they hope and pray that society will show them more tolerance. Rather, they glory in their sufferings, and they think that they share Christ's passion when they suffer. One purpose of suffering is that it builds character (Romans 5:3ff.), but was there a reason that Paul and Ignatius had to suffer on behalf of the church? Was atonement part of this reason? If so, that wouldn't nullify Christ's atonement, since the New Testament is clear that Christians still need forgiveness, even after they've been cleansed by Christ (Matthew 6:12-15; I John 1). Also, this model treats atonement not merely as an individual affair, but as something with communal relevance.