Is there a way to search through all the players in the world by rating?  For instance, if I wanted to search for the highest pitch calling rating out there, to propose a trade, is there a way to filter this? I can't seem to find it...

In general, no.  But for the special case you're talking about, you can use World Office/Reports/Player Search, then choose ML/All franchises/Catchers in the dropboxes.  Because catchers are so different from all other players that they're essentially always going to have catcher as their primary position, the player with the highest PC in your world will certainly be found on that search.

The general problem with that strategy is that (a) players often don't play their labeled primary position (or wouldn't for you) and (b) the Player Search function will reject "all players" and "all franchises" as too large a search, even for the ML level only.

thanks