My suggestion to Comment Portability: The Commenting Standard of the Future is a very simple, tried an decentralized solution:
Add an extra "comment tracker" URL to any comment form and trackback to this URL the content of the comment, the link to the post, etc. It will be much like blogs, some users may prefer a hosted solution, others may set up their own comment tracker. It will look muck like a blog, if not integrated into one (my comments on any blog could appear as posts, or asides on my own blog too).
Most of the code to implement something like this is already out there, included in almost any blogging platform. The tools, like akismet, to deal with spam, misuse, etc, are also there, and so are the tools to export, backup and migrate the data.