I'm writing this up as a gist, because I'm not sure I'll pursue any of it, but it seems worth writing down.
A basic problem with IPv6-only LANs at the moment is that you still need support for legacy protocol servers, which implies you doing some kind of DNS64 and NAT64 (IPv6-to-IPv4 stateful translation).
Currently, NAT64 only exists outside the kernel, as userspace software. This is not ideal because it limits available performance (since you have to keep bouncing between user space and kernel space),