I've decided to go through and list all the windows built into the default UI that never show up without using /show_window, as well as listing some info about each.
Format:
Module.Windowname
Does it contain dynamic information (or is it useful in its current form), and can it be automatically shown by OnHide="visible=true"?
Comments
Let us begin. If you see anything incorrect or incomplete, have more windows to add, or want to update an "Untested" or "Unknown", post it.
MainHUD.Clock
Yes, Yes
Good old sundial. On a side note, the game to real life time ration is 20:1.
MainHUD.Character
Yes, Yes
Only useful information that can't be found elsewhere is located in the Spell Effects text object. Quib already has a mod for this window that cuts out everything but that object and its scrollbar.
MainHUD.QuickBar and MainHUD.Memorized
No, No
Old HotKey system, similar to EQ1.
MainHUD.RestartTrialButton
Yes, No
MainHUD.ButtonbarPopup
Unknown, No
I'm thinking that this may be tied to Inventory.Highlight, the arrows that spin around an ability icon on the hotbar when you first get it.
MainHUD.JunctionChoice
No, No
The old Death window.
Inventory.Examine
Unknown, No
This isn't used, the real Examine windows are in their own module.
Inventory.HTML
Yes, No
Semi-functional HTML window.
Popup.Questionnaire
Yes, No
First used sometime in the week of March 14th.
Popup.PopupInstructions
Unknown, No
Appears to be a duplicate of the tutorial window MainHUD.Instruction
Popup.Book_Simple
Unknown, No
Popup.ScriptExecutor
Unknown, No
Might be GM stuff, might be our UI scripts. For some reason I'm scared to touch it.
(Update: Can't get any sort of UI scripts to do anything.)
Global.Console
Unknown, No
Same story as Popup.ScriptExecutor. I feel afraid to experiment with it.
Option.OptionCalibrate
Yes, No
Comes up on initial install(?). Doesn't come up with /show_window. Haven't seen a way to bring it up afterward.. Not really useful as a window to hijack anyway since the entire Option module is one file.
HUD.Disconnected
Unknown, No
I'm not sure, but it seems like some other window is used when you are disconnected. Or this might be exactly what it appears.
HUD.Mouselook
Unknown, No
PlayerHousing.PurchaseHouse
Unknown, No
PlayerHousing.PlayerHouse is the window that actually handles the purchasing and rent payment. (Can't seem to get /show_window to bring up anything in this module.)
Examine.Examine
Unknown, No
Don't think this window is used, all the Examine windows I've seen fit the other windows in the Examine module, or MainHUD.ReplyDialog (books, collection items).
IME
Recently added, probably not fully integrated yet. Can't even get it to show with /show_window.
Popup.Loadskin
NO, No
Since it's not actually of any use at the moment, might as well see if it can be hijacked.
The point:
It appears that there are no windows to hijack for our own custom windows that can be made to automatically show themselves, except that they are already used for something useful. Unless you count the Typo Report window.
Luckily, they can be hijacked without altering eq2ui_mainhud.xml or anything else that generally should be avoided.