I took me a little bit to grasp what you were actually asking.
How does the positive pressure effect the modulator valve?
I ran a stock C4 in the green car very briefly and I don't remember having any issues.
The modulator valve is used to help pull the trans into first and help hold during a shift. The biggest thing would be to make sure you hook up the kick down linkage. This takes your trans operating pressure (line) from below 100psi whatever peak pressure for the pump is (220psi). Since line pressure is what does the work and holding its probably the most important.
I would just put a check valve in the vacuum source for the mod. valve if there were any problems and you'd probably be good to go.
On the other hand, and full manual reverse pattern valve body can be bought new for $150 or less.