As you say, there are no plugins to enter /administrator.
If you want to write one, you need to create a system plugin and login component.
To catch the login, the plugin systemfor onAfterRouteor onAfterDispatchwill need to configure the destination to indicate your own version of the version /adminstrator com_login.
com_mylogin - com_login, , .
, - , - . Captcha .
, :