Data point from a week and a half ago regarding LAX and MIA Admirals Clubs:

Saw somebody (Pax) popping out of the shower at MIA. Forget what the live station was in the morning and it's a month since I was there at night but they had something going then up to when I left around 9:00 pm.

Was talking to the Club Manager at LAX and she told me that they'd been opening up the First Lounge side for a couple of weeks when things started to fill upon the AC side. Per the Manager, they haven't opened the whole First side but they're been opening sections, based on demand, starting from the south end where the passage to the AC Bar is and heading toward the First Lobby . When I was there they've opened as far as the couches across from where the Freestyle machine used to be. My guess based on what I was told that they'll be completely open (up to the glass wall/door to the First Club Lobby) for overflow by mid April assuming things keep opening up.
The Manager also said that if the traffic keeps going up then First (and maybe Flagship) will reopen based on the numbers of people. No idea of when First will be open as full First.