Don't you still need a loop that is above the waterline when the boat is heeled to keep the holding tank from emptying back out the head while heeled? yuck
I would not want to find my cabin sole covered with poop, would make for a miserable sail and I know who would get stuck with clean-up duties. The porta potty may be a simple $125 fix in the short run.
PS evaluated several recently, they have come a long way and they don't use as much water to flush.