Nice idea with the taskbar.
Have you tried what happens if you do:
<window>.visible=false
show_window <window>
This won't reset the window, but it might not make it visible again.
Otherwise: Is it an option for you to work around some windows resetting by copying their data/setup to a temporary data object before doing /hide_window and then restoring it when doing /show_window?
Setting the taskbar up as a TabPane is another idea, but probably not what you want to do. Also, I suppose you'd have to place that TabPane in eq2ui_mainhud.xml or higher and I'm not even sure if that works.
EDIT: Ah, you posted while I was typing. Looks good your screenshot. I should do a button design like that!