If a passenger steps in for the crew, are they eligible for a reward as in maritime law?
The short answer is no but it really depends on what kind of "pilot" is on board. 


  "Is there a pilot on board"


If the pilot in question is a Private Pilot as per FAA licensing (although its often similar elsewhere in the world). They would be legally prevented from receiving compensation for flight time as per the FAR's. 


  § 61.113 Private pilot privileges and limitations:
  
  (a) Except as provided in paragraphs (b) through (h) of this section,
  no person who holds a private pilot certificate may act as pilot in
  command of an aircraft that is carrying passengers or property for
  compensation or hire; nor may that person, for compensation or hire,
  act as pilot in command of an aircraft...


If the pilot in question was a commercial rated pilot they could in theory receive compensation for the flight time but as far as I know there is no aviation law like the maritime law.

Furthermore maritime law generally allows you a portion or all of the recovered cargo value (think treasure salvage) in the case of a hull loss. Here in the US if an aircraft were to go down be it over land or water it becomes an accident scene which the NTSB would have jurisdiction over and is generally in charge of finding and salvaging for their investigation. If you were to recover a piece of an aircraft that was part of an ongoing investigation you may be required to turn it over to the authorities.  

The closest scenario that I know of like this, was when a bomber pilot helped land a plane after one of the crew members had an issue. However he was not pilot in command and was only providing support. 

All things considered you will probably get a book deal out of the whole thing if it were to happen...