Dynamic Oracle Pivot_In_Clause
This may or may not help you, but with PIVOT XML instead of the simple PIVOT you can do what you want.


  Adding the XML keyword to the PIVOT operator allows us to convert the
  generated pivot results to XML format. It also makes the PIVOT a
  little more flexible, allowing us to replace the hard coded IN clause
  with a subquery, or the ANY wildcard.


(source)

This will return obviously an XML, this is why it may not help you...