There is no built in mechanism to do that. You can compose the set of data files based on facts (instead of filtering - you compose from parts), if that does not work, you can (not recommended) set a global variable to the fact filtered value, or write a small hiera backend function that does the filtering for the specific key (looking up the data to filter under a separate key). While doable, why do you need this? Most people don't need to jump through hoops like these.