So from a regulatory standpoint, 121/135 experience can only be accrued at an actual FAA certified 121/135 operator?
I'm not sure what you mean by that. What FAA certificates, ratings or privileges require a certain amount of 121/135 experience?
Let's go back to your first example:
If it is legal there to log SIC in the right seat of a King Air 200 due to the OPSPEC, would a US airline count the time? Basically what bearing does flight activity logged overseas (while flying under foreign rules) have when using it to show experience in the States?
As already said,
a US airline might or might not count the time as valuable experience for the job. But that's not "regulatory." "Regulatory" is whether, for example,
the FAA would count the non-stick time in that King Air toward, say, the 1500 hour requirement for the ATP, or the 50 pilot hours/100 cross country hours required to act as PIC in a Pat 135 operation.
The answer to that one is probably no (there may be some agreements or exemptions floating around I don't know of). That's because to count time toward FAA certificates, ratings, and privileges, the FAA's rules on counting time need to be followed. That seems pretty straightforward.
OTOH, it works both ways. There's no requirement an airplane be registered, located, or operated in the US in order to count flight time. If what you did meets FAA definitions of what counts (is loggable), Canadian rules don't matter either. Sticking with that King Air, which requires no type rating, all of the time you were the pilot flying is countable as PIC flight time in the US, regardless of Canadian logging rules.