Well i create a folder called "MyUI" under the EQ "UI" folder then i set the eq.ini with "cl_ui_skinname MyUI".
Next i copied the default player xml file into the folder and did the modification there.
I did tried using /loadui and doing the logout then in method but both gave me the same results.
If i change some properties /loadui and it disappears, changing it back to the original properties, doesn't work make it appear again.
I had to replace the whole xml file to make it work.
Sigh ...
Edit: Ok, latest update is i downloaded the UIBuilder here and tried it. This time i set the "tick" to hide using the visible option like you recommended. Login to the game and the Health bar disappear still (i only tried on the health bar)