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> by using throwaway addresses for each connection
This is technically possible. I've seen some services using this technique to bypass Google search query limits by switching their egress IPv6 address every 5 minutes or so. Of course this works only as long as Google doesn't enforce limits per subnet but switching is easy. Choosing a new IPv6 address *per connection* is also technically possible with /etc/gai.conf, the client software would just need to actually do it.