My desire is to get system generated mail (currently delivered to the file /var/spool/mail/root) to be picked up from another machine by user peter
So that means you should deliver/redirect this email to peter (or you need to start messing with shared/public mailboxes)
Is there any reason that you are using system users?
Linux authorisation/authentication is the best, you don't want to rely on crappy virtual user implementation of some developer who's core business is not even auth.
Does a console user with account 'peter' actually need to have access to the mail on the machine running dovecot?
Where does he write a system user is a console user?
In general, I find a setup using virtual users a lot easier to manage: no filesystem problems like the one you have, as all mail is handled using a single uid/gid (named vmail/vmail or so).
What he does here is good. Not wanting to do filesystem permissions because you can't set them up correctly is not really the way to go. Security is always based om multiple layers