Quite a few recumbent cycles have a long drive chain that goes under the seat; and in many cases the line from the top of the front spricket to the top of the rear sprocket doesn't.
In those cases the most common solution is an idler, on the tight side (and often one on the loose side as well).
How much loss does this arrangement make? It would not be too hard to run a pair of chains and a pair of sprockets where the idler is placed - wouldn't that be better?
I don't know your answer, but I have seen data on friction losses through a derailleur. It ranges, according to the manufacturers, from 1% to around 5%. I once had an early Campagnolo Record, which was steel jockey and tension pulleys, with ball bearings and adjustable cones on both pulleys, and they claimed 2%.
You probably would have less because there are no sharp angles.
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