The trailing edge of the rudder should hopefully have some pressure recovery, so possibly comparable or slightly higher than cabin pressure. Even so, the 'slip stream' flow would still probably carry any liquid away efficiently. Probably want to test with water first.
In addition to the gravity pooling, another concern for such a long run of tubing is that it can freeze, plugging the tube. I had that happen in a glider once while descending out of wave. Was still at 20,000 ft or so.
Many glider guys now have a 12" tube that can be extended down through the belly through a rubber grommet. They retract the tube when not in use, and extend it for use. You are right that the propwash would make a mess of that idea.
"The Magic Carpet" Flying since Sept. 2009
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2021