I am using Magento 1.9.2 and often customers checkout as guest because they are involved in a process and do not want to create an account in the checkout. I want to set a second possibility on the success message page after place the order. the customer is relaxed and maybe it's easier to get a new reg. customer then. any help appreciated.
Here are a few ideas.
Have you considered just adding a large button on your /app/design/frontend/yourtheme/default/template/checkout/success.phtml page with a link to your account registration page?
Maybe running a script for a pop-up would also work if you redirect your checkout/success page to the home page. At least the customer would have to interact with the pop-up unless they are running a blocker.
Make registration the only option.