I've only seen VisualSVN authenticate users through an active directory or LDAP. But I don't see why you couldn't point VisualSVN to authenticate through the localmachine. In that case you wouldn't actually do this through VisualSVN, you'd do it through the OS of that machine. On a Linux machine, you'd add a user, then add that user to the svn group. On Windows you probably just need to add the user.
Then in the VisualSVN command window, you'd right click on "Repositories" (top level artifact) and select "Properties", then click "Add". Ensure "Object Types" has "Users" selected, select your machine from "Locations", and enter the username. All repositories will inherit this permission by default giving that person access to everything.
To automate it further, create a user-group on your machine, grant access to that group, then just add a user, and assign the user to the group in your script. Then you'd just need to give the group permissions and you'd never need to touch VisualSVN.