Parameter Field Elelments = List of Directories

I have Simple Image Gallery Pro from JoomlaWorks.

I also have K2, but have been disappointed with how it handles files, so I am looking at FLEXIcontent as the main content handler for my site (which is currently in BETA, so I'm running out of time, LOL!).

I understand the concept of creating the plugins, I believe.

What I'm not exactly clear on is how to create a dynamic list of choices from the drop-down of a certain set of directories/folders on my site.

For example, with:

{ g a l l e r y }folder_name{ / g a l l e r y }

We call the plugin. If folder_name is a folder in the images/stories/galleries directory, how would I make that a dynamic choice, so that I can choose from the galleries that exist?

OR, make the field a write-in field, to type the folder_name in, without editing the field for each?

I plan to have 1 new entry each day, so eventually, there will be quite a lot of choices...

I also thought about hard-coding the plugin into the specific template, and using a field to fill it in... but ideally, there is another way.

Thank you.