That sounds easy, but I think you just inadvertently demonstrated the quagmire that many seem to perceive.

Suppose I've got a machine on the LAN and I want to open it up to the world on port 1025 with IPv6.

This can't happen with SLAAC? It has to be a new address? Does the ISP pick the prefix for that address? And one can't centrally-manage that address (because people keep saying that DHCPv6 isn't worth stuffing around with)? What happens when the router fails over to a backup ISP? How does dynamic DNS fit in with all of this?

Those questions don't really exist with IPv4, wherein: One can just set up a static DHCP assignment, forward port 1025, and [optionally] set up dynamic DNS -- and this all happens within the confines of a single home router.

If it's MAC based SLAAC, it'll be stable, so you could use it. Otherwise you add a stable address. You could use DHCP if you want. If you're using a backup ISP with dynamic DNS, your failover should be the same as ipv4. You can already have ips from both prefixes on the interface. You're also way past clueless home user stage if you have that setup at all, so I can't imagine ipv6 is the stumbling block.

Things would be nicer if NAT66 were used by default for home users though so the question of prefixes would disappear, and it'd perhaps match the "more advanced" home user ipv4 mental model. You'd just use e.g. fd00::2 as your server address.