There were two main things that I had to do to make this work across reboots:
The path "/root/.ssh" is a symlink to "/boot/config/ssh/root/" -- so, any keys added to /root/.ssh/authorized_keys should persist across reboots.
Copying "/etc/ssh/sshd_config" to "/boot/config/ssh/sshd_config" and only editing the copy in "/boot/config/ssh/sshd_config". This way, changes to the sshd_config file should be persistent across reboots as well.
In my particular case, the actual changes I made to "/boot/config/ssh/sshd_config" were the 3 lines below (which already exist in the file; I just uncommented/changed where needed):
PermitRootLogin prohibit-password
PubkeyAuthentication yes
PasswordAuthentication no
Once the changes to "/boot/config/ssh/sshd_config" have been made, you should be able to just restart the SSH daemon via "/etc/rc.d/rc.sshd restart". There's a section in that script that copies the contents over from /boot/config/ssh/* over to /etc/ssh/* (minus directories). Search for "sshd_start" in the script to see the actual section that does this.
This has worked for me & I've been using this setup for quite some time.