intuition

If it is a oneworld branded lounge, I would expect the staff to adhere to the principle of the alliance.
If that would mean I would be turned away, because the lounge actually was full when I arrived, so be it.

If you arrive at a finnair lounge in HEL and it is full, do you expect them to toss someone flying JL so you can enter?
A lounge is a first come, first serve type of establishment. It is not a pre-booked restaurant. If it is full, it is full. Is it worse if it is full with one kind of eligible flyers and better if it is full of another kind of eligible flyers?

If a lounge is full (actually full, not fake full) and is so consistently, the lounge operator could opt for a larger lounge space as they clearly can attract more travellers (and thus more revenue from partner airlines). Sure, not all airports have more space. But with the BA in NA situation it is not about space. There is ample space in a thrid party loung where they tell you to go. It is simply that they don't want you in their lounge. That's not cool.