I made one similar to the triangle thing. It was noisy as hell. Look at the side profile of a chain and see how it goes up and down. An image.
http://www.bikepro.com/products/chai.../cc_slt_99.jpg
The up and down profile of the chain meant it got tight, loose, tight, loose which caused it to vibrate and make a lot of noise.

You would need some special pulleys/rollers that where designed to fit the shape of the chain. Perhaps some weird recumbent pulleys would work. I'd guess they are the only situation on a bike where the section of chain that transmits the driving force would pass over a pulley/roller.