Of course, you'll get a bunch of warnings from the activation script:
$ nixos-enter --root /tmp/mnt/
setting up /etc...
mount: /dev: permission denied.
mount: /dev/pts: permission denied.
mount: /dev/shm: permission denied.
mount: /sys: permission denied.
/nix/var/nix/profiles/system/activate: line 74: /proc/sys/kernel/modprobe: Permission denied
chown: changing ownership of '/run/wrappers/wrappers.0pKlU8JsvV/dbus-daemon-launch-helper': Invalid argument
NOTE: Under Linux, effective file capabilities must either be empty, or
exactly match the union of selected permitted and inheritable bits.
Failed to set capabilities on file `/run/wrappers/wrappers.0pKlU8JsvV/ping' (Operation not permitted)
chown: changing ownership of '/run/wrappers/wrappers.0pKlU8JsvV/unix_chkpwd': Invalid argument
[root@nixos:/]#
da702a4034