what about hooking it directly
Lua Code:
local function newSetHidden(...)
--origZO_PlayerAttributeHealthBarLeftGloss(...)
end
local origZO_PlayerAttributeHealthBarLeftGloss.SetHidden = ZO_PlayerAttributeHealthBarLeftGloss.SetHidden
local origZO_PlayerAttributeHealthBarRightGloss.SetHidden = ZO_PlayerAttributeHealthBarLeftGloss.SetHidden
ZO_PlayerAttributeHealthBarLeftGloss.SetHidden = newSetHidden
ZO_PlayerAttributeHealthBarRightGloss.SetHidden = newSetHidden
that's a sample of course but that's how you can change the SetHidden behaviour after you changed it once