If swallowed by a creature with the Swallow Whole ability, can a character make Called Shots (from Ultimate Combat) to vital organs like the heart?
Most creatures aren't hollow bags with floating organs in them waiting to be stabbed. : )
Called shots are an optional mechanic and subject to a lot of GM discretion. Swallow Whole, on the other hand, is more well-defined--the swallowed creature is grappled and its movement limited enough that only light slashing or piercing weapons can be used to escape. I wouldn't personally allow called shots while in such a situation, but YMMV.
In general that's going to be harder than making a called shot to the heart from the outside. You have some idea where the heart is when you're outside the creature looking at it, but from inside the stomach?
Plus what blahpers said.
Best case scenario for you, if Called Shots are being used in the first place, it would at best use the published rules and wouldn't be any easier.
It's kinda funny. In my current game, one of my fellow party members got swallowed twice. Both times we were out on a boat in the middle of the ocean - first a megalodon and then a giant kraken. We were able to get him out before he died, but it's still a funny situation. While he was inside the creatures, the GM determined that he could attack normally as long as he had room to move and would be rolling against touch AC.