Can an S-duct fit a turbofan exhaust instead of the intake?
Yes they can, and it has been done in the Garrett AiResearch ATF3. Although the engine was not hugely successful, it is still being used in aircraft today.



From this answer:


  The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon.


You can see several S-ducts leading the hot exhaust gases internally into the little staircase-like contraptions behind the fan.