Edit: Nevermind, I looked at the Fetish one again. Interesting...
Any other custom skin modders: WC_Backdrop needs to be set to
visible="false" and the script set to trigger
OnShow with no OnHide event.
The game forces WC_Backdrop to always be visible. If you don't make it visible yourself then the game will force it to show after the main window's script events, but if you DO make it visible yourself with
OnHide="visible=true" as is customary, then the script executes BEFORE the parent window's.
At any rate, that didn't take nearly as long as I thought it would.
Here's the MookUI EQ2MAP skin. Its official release will be after the modular version makes it to the download section.