Nice design. I've made and modified similar setups myself for other engines before. If you have a good baffle setup it should work nicely.
If you are setuf for full time drainback into the pan, you can go to an Isuzu truck dealer and get a nice one way valve from their factory oil breather setup that works nicely to keep the oil flowing downward but keeps the gasses from blowing back up into the oil separator unit
You can also get a one way valve from Racor, they use a nice one with their CCV4500 breather filters
That one way valve will keep the pressure down in the unit so the oil will go where it's supposed to