Tapped backplate runs fine, you can open the venturi.  Recommend a one way valve in pressure line, and uniflow tank.  Suggest you stay with bladders if they work to suit you.  Just plug the tap.  There may be a one way valve in the tap. 

I would think the crank pressure would give more pressure than backplate pressure.  Backplate pressure changes from positive to negative, and back again as the piston goes up and down.  Crank pressure should be timed to give only positive pressure.  I always cut the nipple off as it gets in the way of my motor mounts. 

My theory of crakcase pressure is this:

On the downstroke the piston sends A LOT of pressure into the tank.
On the upstroke it sucks a lot of air in through the venturi and allows a minimal amount of pressure to escape the tank.
But the net result is still PLENTY of pressure in the tank.
More than enough to drown any engine in fuel, even with a giant venturi.