http://www.eq2interface.com/forums/showthread.php?t=229
you can get the plugin there. All the files with a .dds extension in the /images folder are the UI images. The DDS extension is short for Direct Draw Surface, the plugin will let you open them in PhotoShop to mess around with 'em.
I also heartily recommend the DDS Converter tool from the nVidia website, you can use it to do batch converts of a whole directory of images, should you have the desire.
Hope that helps. Keep in mind, truly custom UI's use art that was never originally shipped with the game, so anything you can create or get the author's permission to use, could conceivably be made into a UI.
Wulfy