Ok so I've tested a little more, seeing I also like the non-glossy version
Lua Code:
ZO_PlayerAttributeHealthBarLeftGloss:SetHidden(true)
ZO_PlayerAttributeHealthBarLeftGloss:SetDimensions(0, 0)
ZO_PlayerAttributeHealthBarLeftGloss:SetDimensionConstraints(0, 0, 0, 0)
ZO_PlayerAttributeHealthBarRightGloss:SetHidden(true)
ZO_PlayerAttributeHealthBarRightGloss:SetDimensions(0, 0)
ZO_PlayerAttributeHealthBarRightGloss:SetDimensionConstraints(0, 0, 0, 0)
Seems to work and only gets called once. It avoids the re-showing problem by just setting the dimensions of the glow to 0 width and 0 height.