I have in the past on some custom construction stuff. It "works" in the most basic of sense.
Here's the problem. You wall isn't flush. It's just not. When you screw drywall on the studs and mud over them you are left with irregularities in the surface. The same is true of taped joints between sheets which, if you're going to make a fairly large screen, you are invariably going to cross. You can sand and sand all you want but you'll never get a truly smooth finish.
Now, I hear what you're thinking. "But Skip, my walls don't LOOK irregular!"
That's a trick of the paint. If you were to use a high gloss paint, which is what a paint on screen basically is, you'd notice these effects immediately. It's not a sheet of metal or plastic. It's a porous, chalk based engineered material. You'd likely be better off bolting MDF and painting that.
You can get a relatively inexpensive screen used. Many screens are designed to be snapped together and pulled taught then bolted to the wall about 1/2" off the surface. So it's not as if there's a ton of room involved in these things. The screen would be held about 1/2" off the wall, just enough to avoid the wall touching it and causing irregularities in the screen.
OTIKA, I'm not trying to rain on your parade here. But I want you to have the best information possible so you can make an informed decision. I shopped for projectors for 2 years before deciding they're not what I want. There's a HUGE difference between a cheap $500 Power Point projector and a quality home theater projector. You owe it to yourself to consult some local home theater stores and see the difference. I also highly recommend checking out
AVS Forum. There is a wealth of DIY information there to pour through.