Hi, AFAIK this is not possible directly, except you can probably use a .alter configuration file to accomplish this if you have already rolled out the devices.
If you are still making the default config, you could include something like changing the admin password to the router serial number as part of the default config.
One way you could do this is by setting the username and password via TR069 for something that allows a username and password setting - for instance, PPPoE client. Then you could run a .alter script to set the username and password for router login based on the PPPoE client info. This might work for you whether or not you actually use PPPoE on the router - you could potentially have a disabled PPPoE client on the router to hold the username/password.