I don't have any OSX boxes, but wouldn't bind-mounting the real the /some/where/nix to /nix solve the realpath problem? No need to write to / either (as you would have to do with the symlink approach).
If it is possible, then I think it should work. I believe this might be the way they are planning to go, because someone mentioned about a separate partition.
Honestly, I think they should just relocate it to a different location that can be persisted and rebuild the packages. I don't think people care about the location as long as they don't have to recompile every single thing. This could also be useful to catch all bugs where /nix was assumed.
Ideally the Nix store would be in some non-root location. But that requires a completely new binary cache among other things (since /nix store paths are hardcoded in binaries, scripts, etc).
Also making /nix a symlink doesn't really work in some cases, since realpath reports the actual path and that may break builds/applications.
It used to work so nicely out of the box :(, but I can also understand why Apple wants to enforce read-only system volumes, since it blocks nastier rootkits, etc.