Re: So they want..
"With ipv6 they can do a 1-to-1 mapping of IP address to device, which surely is much stronger from the point of view of bureden of proof."
not only that, but an IPv6 user is likely to have an assigned netblock, which "identifies" you. So, in actual fact, it's EASIER to tell who you are, because your netblock won't change.
As I recall, I've got two /64 blocks assigned to me. that leaves about 2**60 netblocks for everyone else, assuming that we're all assigned netblocks from 2000::/3