You lost me at “multiple per-interface NAT rules […] with some load balancing trick” being “stupid easy”… But BGP+PI is indeed how multihoming is supposed to w

You lost me at “multiple per-interface NAT rules […] with some load balancing trick” being “stupid easy”… But BGP+PI is indeed how multihoming is supposed to work, both in IPv4 and IPv6. (Well, in IPv6, you can put two prefixes on-link and do poor man's multihoming that way, which you cannot reliably in IPv4.) Of course, if you define “it cannot be BGP+PI”, then indeed it's probably harder, but if you exclude the intended solution, obviously there's no intended solution.