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Isn't about the same achievable with about two or three lines of scripting (or a new zone parsing option for bind ;) with a lot less protocol complexity?

only if you can tolerate short TTL's on all your AAAA's. in the A6/DNAME model, your A6's could have long TTL's whereas your DNAME's could have short(er) ones.

As you note, A6/DNAME wasn't a panacea. A lot additional stuff is needed to achieve the goal. It seems to me that actually the A6/DNAME part is a relatively simple one to achieve using current mechanisms.

the other issue is multihoming. someone who got done traversing the maze of A6 and DNAME RRs that it took to find your addresses would pretty much know that you were supernetting at the LAN level and that they should use a very short timeout when connecting to each address. when someone gets back multiple AAAA's for you, then you might be multihomed, and folks will do just what they do with multiple A's, which doesn't support rapid renumbering.