Okay, I just did this mod today. You can see the beta pic in the beta forum.
Pet_HP is tied to the PlayerWindow. It *must* exist in the <Pieces> in the Player Window.
What *must* exist is just this: <ScreenID>Pet_HP</ScreenID>
Copy the Pet HP gauge and move it to you other window. Change the original to size 0,0.
Item= can be anything you want, but it must contain <ScreenID>Pet_HP</ScreenID> inthere some place.
Add it to the pieces and work on the rest until your heart's content.
FYI - <ScreenID> is VI's original form of EQType. Apperently it was converted to EQType, makes since, but VI didn't remove all the ScreenID requirements from the game, suckage.
So, your custom additions can have any ScreenID, or none, but any current one it's preffered you leave them intact.