If the fluid is thin enough, it might. But with a lot of illnesses, the fluid is thick and sticky. It would be like trying to drain Silly String.
Having your head below your feet for long periods would be uncomfortable, but having your head below your rib cage might not be as bad, and would have the same effect on the lungs.
I have some back exercises that result in that, and I can’t say I’ve noticed any effect on my lungs. But I’ve also never tried it when my lungs were congested, either.
If you decide to test that method, please let us know whether or not it works.
If I get that flu, I will. Thinking you lie on your stomach, with your head above a breathing hole. Like a massage table. Then your feet side are lifted high. Dont forget a bucket. Lol.