For chain adjustment, could you use a 10-speed bike's deraliur? it is spring loaded, and would keep it in decent tension, but not great. I used this type of set up on a 3-wheeled recumbent bicycle that I made for a kinetic art race. I imagine that you have a slot and bolt with either a wheel or a sprocket on it.
I saw a recumbent bike one time that used a wide skateboard wheel as it's tensioner. The guy had so many miles on the bike that it had formed itself into a pulley shape, and the bearings were still good.