Beautiful bike (I posted earlier but my post seems to have disappeared)!
The two big ports on most Kz/Zx motors are not crankcase ventilation ports. They are simply ports directly into the exhaust stream. So a tube from elsewhere in the exhaust system going to those two ports won't really do anything.
Also, how do you know that the 4-into-1 collector will always be at a negative pressure relative to the crankcase? Under hard acceleration, I would expect higher pressure there unless you have no mufflers at all. I've seen exhaust systems with leaks there and the soot marks clearly indicate that at some point, there is positive pressure there (since the exhaust is puffing out of the leak). You could alleviate that possibility with a check valve between the crank vent and the exhaust, and another check valve from the crank vent open to atmosphere. Obviously anytime you are potentially pressurizing the crankcase, you run the risk of blowing oil past all the seals.