Jetcap,

Just a couple points of order...

135.113 refers to a pilot seat, ie left, right, or perhaps even the FE. It is silent as to the observer's seat.

135.85 refers to who counts as a passenger. ie if my landing lights are inop I can only operate at night without passengers. This laundry list enumerates those who are also not passengers for the purposes of a specific regulation. Again, not quite relevant to who may occupy a jumpseat.

It looks to me like 135.125 is going to be airline specific per each carriers' own ops specs. And those rules are going to be SSI, thus not transmissable over a public message board (or any other means!).

But I totally agree, the short answer is going to be no.