Every server I've set up to use SQL Server backups to a UNC location on the network has been using a Domain User account to start all SQL services; this is a Domain account that is a "user account" (i.e. not a computer account or other special account).
When you do this, make sure you go through permissions and Group Policy carefully; there are several things you need to do to get things working properly (and a couple more if you want basic features like Instant File Initialization).
Other methods may work also. Note that some SAN and NAS devices have an additional security layer over and above Active Directory.