best i could come up with...the tensioners are used to "pull back" on the axle. it's for BMX style dropouts, maybe new downhillers too? very similar to motorcyles.
yes, it would very helpful for that, especially with two chains...if the pedal side is multi-speed cassette, you can eliminate the idler on the engine chain.
I use similar chain tensioners on my StingrayXL. they'll only work with straight back dropouts of course but they keep the chains from changing around really well. I found them in a site for those stand up powerboard scooters because I saw the same thing on mine but didnt want to immobilize the scooter for the chopper.