I'm also interested in seeing if this works. My explanation is based on my experience with hydraulic racks (i.e. Just about every rack that isn't manual). If the Focus is equipped with essentially a power assisted manual rack then you may very well be right. I however will not be the guinea pig for this.
Also I don't know how much experience you have with hydraulics, but the whole point of having fluid in a system is to amplify force for so that it is easier to push/move something than it would be with a mechanical connection. Take for instance the braking system. The brake pedal pushrod pushes into the master cylinder with a very small surface area and the brake fluid transfers the energy to the piston in the caliper. Now for simplicity sake we'll assume the piston in the caliper is twice the size of that in the master cylinder this means that for every 1" the master cylinder is depressed the caliper piston will be pushed to move .5" with twice the force of the input on the master cylinder. Do you realize how hard it would be to stop a car with the force you could mechanically push on the caliper piston? As it is you get a high force multiplier (around 10>1 or more) with an engine operated power assist. Now apply all of that to the rack (this all assuming of course it is hydraulically operated). All of these factors come into play with the inputs on your steering wheel. Why do you think it requires so many turns of the wheel to turn the wheels <90 degrees? Because it is giving you that torque multiplier for easier use. If you tried to replace the hydraulic fluid with air (that compresses) you may only be able to turn the wheels very little, if at all, and though it may not be that difficult it would be useless. If it is a mechanical rack with a hydraulic assist then it will be much more difficult to turn the wheels because you lose that torque multiplier; and based on the design it may use the P/S fluid as a lubricant / cooler. In which case it would burn out the rack. That is where I got my conclusion that it would not work (or for very long). If this rack is designed differently than I have described and will work as you plan on trying then more power to you. I have no problem admitting I am wrong. Good luck man. Hope it works!
EDIT: Honda (as I see you are basing your info on) are designed very differently than Foci. Even their auto transmissions operate in an entirely different way than ours. Just FFT
EDIT 2: Looking at your top link if you go to the bottom of the page it shows what I'm pretty sure it the design of the rack that the focus has. If you try to replace the fluid in a a rack designed like that one with air you will burn it up if it functions at all. This I have seen happen....to my car.
Fast cars make smart people do dumb things