This has led me to follow my own advice and explicitly draw up a list of conditions necessary to affirm that the Johannine aposynagōgos passage are, following Martyn, describing the Johannine community's late first-century experience of being expelled from the synagogue via the mechanism of the Birkat ha-Minim. Of course I identified and investigated the conditions necessary for this to be the case in my dissertation, but I did so in a way that in retrospect seems more haphazard than I would now prefer to be the case. I stand by my judgment regarding the Birkat ha-Minim and its relevance to the aposynagōgos passages; what has changed is that I would now go about arguing that judgment in a different, heuristically more systematic, fashion.
Among the conditions necessary to affirm that the aposynagōgos passages describe the Johannine community's late first-century experience of being expelled from the synagogue via the mechanism of the Birkat ha-Minim are surely the following:
1) The aposynagōgos passages must be referring to events that took place something between c. 75 and 100 C.E.
2) The Birkat ha-Minim must have been operative within synagogues by c. 75 to 100 C.E.
3) Christians must have been the original targets of the Birkat ha-Minim.
4) Given the rabbinic material which Martyn cites to support his hypothesis, the aposynagōgos passages must be referring to a liturgical context.
5) That same material must indicate the Birkat ha-Minim was a mechanism for expelling people from the synagogue.
Let us now re-phrase these as Yes, No, Maybe questions, and seek to answer each in turn.
1) Do the aposynagōgos passages refer to events that took place something between c. 75 and 100 C.E.? 9:22 and 12:42 manifestly situate their narratives in Jesus' life, thus vitiating any attempt to suggest that they refer to events of four or five decades later. Jesus in 16:2 indicates that the disciples will be made aposynagōgos after his departure, thus allowing the possibility that this particular passage might have in mind events of the later first-century. We can thus by way of answering this question offer a qualified "Maybe," the qualification being that for 9:22 and 12:42 the answer must be "No."
2) Was the Birkat ha-Minim operative within synagogues by c. 75 to 100 C.E.?
The general consensus among rabbinic scholars seems to be that the Birkat ha-Minim does not date this early. That said, I cannot say that it is impossible that this was the case. This condition however is not simply about when the Birkat ha-Minim was written but rather when it became operative within synagogues. There is good reason to think that it was sometime later than c. 100, but, again, I cannot rule out the possibility that it was operative by this time in at least some synagogues. Again, the best answer is "Maybe," but with a general inclination towards "No."
3) Were Christians among the original targets of the Birkat ha-Minim?
Rabbinic scholarship seems divided on this matter, with solid arguments on both sides. So, again, a "Maybe" seems to be the best judgment on the matter.
4) Do the aposynagōgos passages refer to a liturgical context?
Here we can give an unqualified "No." In not one of these passages is there even a hint of a liturgical context, and in fact the account in 9:22 would seem to clearly envision a non-liturgical one.
5) Does the rabbinic material indicate that the Birkat ha-Minim a mechanism for expelling people from the synagogue?
The rabbinic material regarding the Birkat ha-Minim tells us that a reader who stumbled whilst reciting the prayer would be removed from the position of reader on suspicion of being a min. There is no hint that the person was thus expelled from the synagogue. In truth we probably would not even suspect that expulsion was involved if we were not already reading this material with the aposynagōgos passages in mind. Thus the answer to this question is a clear "No."
So we are left with three answers that are "Maybe," two that are "No," and none that are "Yes." There is thus no reason to accept Martyn's hypothesis that that the aposynagōgos passages describe the Johannine community's late first-century experience of being expelled from the synagogue via the mechanism of the Birkat ha-Minim and in fact good reason to reject it.
For greater elaboration of this argument, as well as my own historical reconstruction of what is going in the aposynagōgos passages, come and see me at SBL in November. Or read my dissertation. Or both.