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.
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.