I'd use Peter Kahrel's batch_convert script: Free script Batch convert/export InDesign documents | Peter Kahrel - there is a part of the dialog box that allows a script to be run during the opening and converting of the files.
To reach a quick goal (change meaning of 1st radiobutton to "all opened docs" instead of active one):
modify this line (originally no 95...97):
//... case 0: myObject = app.documents.everyItem(); break; //...
above works if some text is selected.
Otherwise aditional line need to be modified (originally no 60-62):
//... default: //Something was selected, but it wasn't a text object, so search the document. myFindChangeByList(app.documents.everyItem()); //...