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?