Can I still boot into Single User Mode if I disable the shell for root? It's been suggested that, since the root account now appears in the accounts database on macOS 10.13.1+ by "default" (by accident it seems), it might be a good security measure to disable shell access for that account.

Will doing so disable the ability to boot into Single User Mode?
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.