I always bring a pair of white sneaker type socks to wear on the plane. They are very thick so I don't think any leakage can seep through from the bathroom floor. At least there's usually only 30 people and not hundreds using the bathrooms.

My feet swell like balloons on planes. I find athletic socks keep the filth off my feet and provide comfort for my swollen feet. I throw them out when I get home.