I think that I have it figured out. I took a peek at the air system on my 4.6L in my Mountaineer.

The PCV valve is plumbed to manifold vacuum (same as a carb). The valve cover air inlet is plumbed to the air intake, so that the crankcase fresh air goes through the MAF. The large hose supplies the air to the Idle Air Control, this is also through the MAF.

I think that I am covered, if I can find a valve cover vent cap that has a hose fitting.
The filtered vent cap that I now have won't work.

Thanks for the help.