Yves,
FWIW, I do not believe your milk is anything to worry about. It is ‘only’ moisture from condensation and is thus eradicated when the engine gets up to correct operating temperature, which shouldn’t take too long for you with 460cc explosions happening each stroke!
But... if you have mounted a breather to the cases in the place provided by Maney (is that correct? Is that where you’ve mounted it?) then you have it mounted in a place where it is an ideal ‘wet sump cure’ as it will very quickly expel any excess oil in the crank cases out of the breather.
Naturally, as breather is not routed to the oil tank, or into a catch tank, that oil is gonna splash out everywhere!
However, when the engine has emptied any excess, I don’t think much more oil comes out of it during normal running. So, if you’re disciplined enough to empty the sump before each ride, you may not need to modify your oil tank.
If I were you, I would experiment with a catch tank to see how much comes out of the breather hose during normal running before hacking your oil tank apart!
I don’t know what a Seeley is like, but I’m guessing that removing the oil tank to modify it is not a 5 minute job? And also, unless the oil tank is baffled correctly, modifying it might create more problems with oil coming out of the tank breather hose!