If you have a bandsaw, drill press or mill, and some student machinists who care about precision, you can make bearing blocks fairly easily. We've done a variety of systems for both live and dead axle drivetrains:
Live axle - We used a aluminum plate with three holes in a straight line, and mounted two such plates with a bearing in each and bolts holding the remaining two holes through spacers and the frame rail. We tensioned it using cams, which were a piece of plate cut in a spiral with a slot for a screwdriver.
Dead axle - We used a rectangular aluminum plate with two holes (one sized for the axle and another drilled and taped for a #10 bolt). A third hole was drilled and taped on end for a #10 bolt. The frame rail had a slot for the axle bolt (which was smaller than the axle itself, and taped into the end of the axle) and a slot for the #10 bolt. The bolt on the end hole was attached to an L bracket for support, and tighting the bolt pulled the block to the end of the slot (tensioning the chain). We would then tighten the other two bolts to keep everything in place.
A properly designed moving idler should not rob any significant power. Make sure the material you use is smooth, an use a bearing if you use a sprocket.
An example of our live-axle bearing blocks on page 4 of the dual drive powerpoint
here. The red fixed cantilever bearing block assembly could be used to build a non-articulating drivetrain, without the green lift forks.