Managing Permissions on a server with new DBs being added
One possible option would be to:


Create a Database Role in each DB (i.e. this should be in your template / source DB) that contains the desired set of permissions.
Create an Instance / Server -level DDL Trigger that fires on the CREATE_DATABASE event and does the following:


Checks the name of the Database
If new DB name ends with ABC then:


Adds the Login to the new Database as a User
Add the new User to the Database Role that has the desired permissions




This will likely require some Dynamic SQL, but it should work. And honestly, I'm not sure what I think of using a DDL Trigger in this manner, but it certainly does automate it ;-).