Mike, great post - and I think the bit at the end is in some ways the most important. There are other ways to go about this, too; I have made the decision to openly sign my paper peer reviews and this has led me to be both more careful and more polite in my reviews, to the point where I feel comfortable posting them publicly once the paper is out.
Two additional thoughts --
I absolutely don't want to see a centralized commenting system come into being for all sorts of reasons; I think we need something sensibly federated. To that end, you might be interested in Chris Lee's "selected papers" network idea, http://journal.frontiersin.org/article/10.3389/fncom.2012.00001/abstract as a way to actually do pre-"pub" peer review in a minimally sensible way.
Second, there are annotation platforms like hypothes.is that I'd love to see applied to this general question of how to (technically) do post-pub peer review. See https://hypothes.is/. Any thoughts as to suitability?