This is very doable, but must be done from an integration. CB does not provide such a functionality out of the box, sorry. You'd need to write a new plugin using CB triggers to execute a JS alert.
You may also want to try editing the successful registration language string and adding something like the following, but doubtful in it functioning.
[code:1]
<script>alert( 'YOUR MESSAGE' );</script>
[/code:1]
Also you've posted in 1.2.1 boards, please upgrade to CB 1.2.3 and use the CB 1.2.3 beards as you're 2 versions out of date.