I did something similar for an image manager script using the dTree[1] JavaScript API. I put a link next to a text form field that opened a pop-up window with dTree listing the directory structure. When you click on a folder it updates the text field and closes the pop-up.

[update]Just to be a bit more thorough: I used a perl script with File::Find to biuld the directory structure for the dTree API.[/update]

~Charlie

[1] http://www.destroydrop.com/javascripts/tree/