One possible option for you would be to have a datapump admin user who will be given the exp_full_database role. Have this user own a package (..or procedure or whatever) that accepts schema names and performs the data export for you from within the package or procedure with calls to the dbms_datapump API. Grant execute privilege on this package to the ids that your developers would be using and that's all they would need (...along with create session ofcourse).

Maybe that's what you were referring to below when you said :

"maybe an equivalent definer-rights stored procedure"
Good luck......

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