while an Ex-Post Facto AWB is entirely possible, it is far from certain that such a law, even if it were to be enacted, would pass Judicial Review.
Heller pretty much closed the door on banning classes of weapons. Even if it were politically possible, I don't see a new AWB (at least one with any teeth) passing judicial muster.
I wouldn't go quite that far. SCOTUS still, by all indications, seems to be OK with the NFA which, through heavy regulation, creates a de-facto ban on entire classes of weapons to all but those with large amounts of disposable income, which is exactly what it was designed to do ($200 was a ridiculously large tax in the 1930's and the Hughes Amendment has driven the prices of legal full-autos through the roof).
I do, however, think than an Ex-Post Facto confiscation scheme would likely push the current court over the edge in what they'd be willing to tolerate and that they would likely strike all or part of an AWB down if such a measure were included.