I contacted customer service today. Here is a summary which leads to 3 options:
This type of "pending" feature is not an option we have available. I will pass this on as a feature enhancement to our Product Management team.
Regarding this work flow though, you can set up your own management system to do this on your server. This type of programming is beyond the scope of our API support, however it would not be overly hard to create a php script that allows you to approve people prior to sending their information to Constant Contact via an API call.