I am concerned that this idea, if implemented, would stick around for way too long and would actually impede the progress of IPv6 adoption. I would be much more comfortable with an idea like this if it had an expiration date from the start, e.g. "this listing mechanism will be considered deprecated after 2 years, and will become unavailable on <date>." Without this, I can see it being hard-coded into and depended on by way too many apps, tools, companies, sites etc etc for years to come, and actually inhibiting IPv6 adoption and causing connectivity issues.
There are loads of issues with outdated blacklists for spam-fighting, for example, and issues with companies and other bodies using their own out-of-date copies/instances of such lists. What would keep problems like that from plaguing this idea, if it was implemented?
Without an expiration date, what would urge these companies to stop using this, or urge others to stop using this instead of offering up content natively on v6 to all?
For the record, I have hosted a large variety of sites (including the official sites for the game Soldat, its related projects, communities, etc) for a few years, including fully dual-stacked DNS and mail infrastructure for over two years with *zero* connectivity issues from any users whatsoever, and that has even been all through a Hurricane Electric tunnel, pushing a significant amount of content over IPv6.