(Tailscale cofounder here) Tailscale already gives every node on every tailnet a globally unique internal IPv6 address, that is reachable even if you don't have IPv6 on the "outside" network. If your apps and OSes are all willing to use IPv6, you haven't had a problem since the early days of Tailscale; they've been solved for years.

Alas, the "apps and OSes are all willing to use IPv6" problem is a persistent one, so we have to make IPv4 work too.