Anywhere there's an opening, an anemone can into it. I dont think your pvc idea would work. There's only 1 safe way, imo. Remove all powerheads and install a closed loop.
Closed loops have intakes as well don't they? How do you stop the BTA from being sucked into them?
If I were to set up a nem only tank like this, could I just buy larger return pumps and use them for flow? I was thinking 30-40x turnover rate so 900-1200gph?
Imo, that's too much without a cl.