Dynamic select box dialog

I have been working with ckeditor lately and ran across a problem i have not been able to solve. Ive got a custom plugin that i wrote for handling image tags. I have a dialog defined for it and basically what i need to do is pull a json string from a hidden text field on my page and use that string too dynamically populate the items: of a select box on the dialog window when my plugin is called. I'm using this as kind of a hacked file browser because my app is written in rails which ckfinder doesn't work with. I'm just stuck as far as getting my dialog.js for my plugin to dynamically generate these drop downs based off of my hidden field. If anyone has any ideas about the way to do this or if it is even possible your suggestions would be helpful.