I am not positive how to do what you want. But I have been using this framework which is very similar to what you have above but is much easier to use
http://aquagraphite.com/2011/11/smof-documentation/
setting up options is really easy and it is supported through github. There is a place to ask these exact kind of questions. That and the framework has a lot of examples in it that you can copy and paste to get your admin area up and running. I just commented out all the example options so that I could reference them when I wanted to create one of them.
As for your answer. I don’t know how to do you but you could try some things like this
"choices" => get_categories( );