I myself tried many of those, but didn't document which ones as I was just in a mad frustration of throwing weird things in. This would be such a useful property if we can just figure out how to use it, that is if it is possible to do this with it. Thanks for trying and I'll keep you up to date with any discoveries.
--edit--
I do remember I tried
eq2ui.MainHUD.Player.BLARG.Opacity=0
eq2ui.xml.MainHUD.Player.BLARG.Opacity=0
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