My laptop uses samba/winbind authentication, so there is no passwd entry on my machine for my user. This would be similarly true on an LDAP or NIS authenticated machine. When I try to use --userns-remap=<myuser>:<mygroup>, I get the following error:

Error starting daemon: Error during uid lookup for "<myuser>": no matching entries in passwd file

I would propose that you should use getent passwd and getent group instead as it would supply details for user accounts which may not be local.