The more I think about it we should be breathing two points in the sump (one on each side)...
The washer solution is a novel idea and I can see it works, but it seems a patch? The oil will still be traveling up the filler tube under positive pressure once it covers the opening in the sump, when it meets the washer the flow rate is decreased (pressure increased) and this buys enough time for the corner to end so the oil never makes it out to the catch can. But during this time the although the major issue is solved we have stopped all breathing to the crank case and would see an increase in pressure, this would be worse in a FI situation for sure?
Imagine the case that both sides of the sump were breathing, in the case the oil covers the fill tube the other side will be open to atmosphere and provide the path of least resistance for the positive pressure, this would mean the oil would not travel up the oil filler at all, the same is true for cornering the other way.
So to solve this problem and prevent other undesirable side effects I think we should vent both sides of the sump...?
PS - Not shooting down the washer solution, I think it is great, just trying to make sure we all end up with the correct solution to the problem.