I own a paintball field and am a professional photographer by trade. When photographing out on the field, I use my least expensive body (20D) along with a 100/300 zoom lens.
The technique i use is pretty easy and only takes a bit of practice.
I use a seperate button on the camera to focus and a seperate button to take the shot. I can pop my focus 3 or 4 times on a target then take the photograph.
As far as viewing, no,,, i cant get very close to the view finder but I can see that my target is relativly centered and indicated as focused. I take the photo and crop the image later using PS to compose a good picture.