It's funny that you mention pressurizing the tank, because I thought of the same thing a couple of days ago, but unfortunately I don't think it would work, because your not just pushing fuel out of the tank, there is also fuel returning to the tank, so if I am thinking corectly pressurizing the tank will not create any extra flow at all because you'd be pushing on both lines with the same amount of pressure.
So greasecar has had problems with their valves failing? That's great to know, I just installed two of them on my car. I was really pissed when I first installed them, because the one valve was defective and had the fuel outputs backwards. So I am driving down the road and can't figure out why the fuel gauge for the main tank (where I keep my veggie) said I used half a tank in 30 miles. I even stopped to make sure I didn't blow a line in the engine, and didn't see anything so I kept going. Then I noticed the trail behind me. Stopped the car and veggie oil was pouring out of my trunk. What a friggin mess, luckily I didn't have anything in the trunk except the diesel tank, but I never did get rid of the smell. I should have made Greasecar pay to have the trunk detailed.

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head