Originally Posted by I_bRAD
Wouldn't the deraileur put too much side force on the chain if it were fixed in place as a tensioner and you're running two different cogs? Never tried it, but seems to me like it would always be trying to pull the chain off. I have used one cog on the back and a MTB triple on the front which worked well. 48X38x28 on the front gives you a pretty large range to work with on the back- regular,easy and 15%grade with a load easy.
I used to run a rear derailleur as a chain tensioner, with two cassette cogs on the back of my winter bike. I locked the rear drailleur in place with a short peice of shifter cable running from the barrel adjuster to the cable fixing bolt. Then I could turn the barrel adjuster to shift between my two gears. It worked nicely, I could get off the bike and shift to my alternate gear in a few seconds.