I built a single speed out of an old 7 speed mountain bike with horizontal drops. NO chain tensioner on this project. I had to use 2 cassettes to get enough spacers to space out the rear hub, but the resulting chainline was perfect. THe cassettes were cheap and were rivetted (riveted?) together. I used a drill and a metal bit to drill out the squashed end of the rivets ( on the back side of the cassette), then I used a small nail punch and a hammer to drive the rivets out of the front side of the cassette. I went from a 7 speed to a single speed in a night, and because all of the parts I pulled off were low end crap, I was able to shave a few pounds off the bike in the process.

I have seen a derailleur set up as a chain tensioner. The guy just dialed in the limit screws to get it to stay where he wanted it (or thats what he said he did).

Have fun with your project.