Yes the registration widget is in v5.2 and no longer 'beta', but it doesn't allow by default for any other options than 'immediately' or 'after confirmation'.
So only route to go is custom unfortunately and the easiest route without totally breaking open the system seems to be by using the 'after confirmation' option.
This will send the user a link by email, which when clicked takes them to a certain page.
Normally you'd place an 'account activation widget' there so the user becomes active, but on this page you could also build your own custom logic to 'postpone' the activation process and for instance sends an alert to an admin to manually trigger the account activation.
Not sure if anyone has implemented something similar or not, but i'm sure if you send a ticket to Telerik they'll provide you with a snippet that'll mimic the account activation.