Is it possible to pass a built in function as an argument to a trigger?
The CREATE TRIGGER documentation says:


  arguments:
  An optional comma-separated list of arguments to be provided to the function when the trigger is executed. The arguments are literal string constants. Simple names and numeric constants can be written here, too, but they will all be converted to strings.


You cannot do any processing before the function is called; you have to put all the logic into the trigger function.