A common emphasis in Bonhoeffer's works such as "The Cost of Discipleship," "Life Together," and "Ethics" is the Church as a both a witness to Christ as well as a blessing to all people. Since the Church is the body of Christ she is called to bring his love to all people. (Goodreads has some good quotes from "The Cost of Discipleship" at: https://www.goodreads.com/work/quotes/2723088-nachfolge)
So, in the context of welcoming non-Christian prisoners into worship, I think this would have been an act of blessing and loving them; i.e. sharing in their pain and discomfort. Christ came to bless all people and made a particular effort to reach out to those who were considered "outsiders" by others. The Church, as the body of Christ, is called to do the same, both as an act of witness to the reconciliation we have in Christ as well as simply to be a source of mercy and blessing, showing the love of Christ through us.
So, in the three options you propose (i.e. worship as evangelism, a belief in universal salvation, a belief in interfaith worship under extreme circumstances), I think it is the first option, although not explicitly so. What I mean is that the Church, and particularly Lutherans (such as Bonhoeffer), trust in the Word of God to bring people to faith. So, Lutherans do not press people to "make a decision" or decide anything. Rather, the emphasis is on God doing the work to bring people to faith.
Bonhoeffer has, however, been criticized by some people for things such as this (because people perceive a universalist tendency in Bonhoeffer), as well as his participation in the plot to kill Hitler (see, for example, http://www.fundamentalbaptistministries.com/archives2/DIETRICH_BONHOEFFER.htm).
From actually reading Bonhoeffer, I think the criticisms of him are due to misunderstandings of what he was actually saying, particularly by those who are not acquainted with Lutheran theology.
A good article that talks more about these issues is at: http://www.ctlibrary.com/ch/1991/issue32/3222.html
A quote from the article which applies to this question is:
"In Bonhoeffer's theology, there is an intimate relationship between Jesus Christ and the church. In his letters from prison Bonhoeffer spoke of Jesus as "the man for others." And in parallel fashion he wrote that the church is truly the church only when it exists for others. Just as Jesus lived his life completely for others (even unto death on the cross), so the church is to serve God by serving the world of need."