Blocking the cooler line ports does not decrease line pressure at all. I don't recommend doing it because you do need a cooler. And a cooler is a must with 3200 or so stall speed, if driven on the street. Running a cooler is essentially a loop unless the lines are too small - it's just a longer loop. There is a bypass valve in the pump that internally connects the "cooler out" and "cooler in" fittings. It opens at less than 10 psi. But that's cooling circuit pressure not line pressure. Cooling circuit pressure is usually very low unless you stall the converter against the brakes (or a transbrake). The pressure goes very high when the converter is stalled and the output shaft is stationary or very slow. But it still doesn't go to line pressure, and line pressure never changes unless the pump is oil starved.
You did make one correct point. The seal at the lower end of the pickup tube doesn't need to be a good one, except that you lose any filtering. One thing that could go wrong though is that if the filter (or whatever you use) isn't correctly attached to the pickup tube, it could block the pickup tube, and that would be a problem - as you said.