Can I still boot into Single User Mode if I disable the shell for root?
As @fd0 already commented, "single user mode uses /private/etc/passwd for login authentication, not Open Directory":
pse@Mithos:~$ grep ^root: /etc/passwd
root:*:0:0:System Administrator:/var/root:/bin/sh
but
pse@Mithos:~$ sudo dscl . -read /Users/root UserShell
UserShell: /usr/bin/false
So single user mode will happily read the content of /etc/passwd
and start a root
shell with /bin/sh
even if you set the shell to false with dscl
.