How to force frame:Hide() to work upon login?
 

I've got a little addon that does MainMenuBar:Hide()

It used to do MainMenuBar:SetAlpha(0) but it kept showing cooldown ticks and it was very distracting so I opted for Hide(). Problem is, it doesn't work immediately upon login and I have to run the chat command I have created within the addon and associated with hiding.

How to supercede whatever Blizzard UI is doing that prevents this from working immediately?

You could use a timer and execute your code a few seconds after login.

On event
 

I would try a On Event Player_Entering_World ect.