Jim Jensma, proprietor of
"Stars" Guitars in Cedar Rapids, IA, made an Ebony arm rest for his Lebeda F-5 that is a lot like the Doug Edwards models at minnedolin's link. The one Jim made sits on two cork-padded feet like a violin chin rest and uses clamp brackets from a violin chin rest. The arm rest clamps only at the very edge and, like the Edwards, is cantilevered so as not to touch any part of the vibrating top.
As mentioned, the main advantage of the arm rest seems to be that it allows correct picking posture without needing to cock the wrist so it isn't as tiring to play for long periods. Of course it also protects the instrument.