Thanks for your request. I'm afraid at the moment there is no way to achieve what you need. Your request has been linked to the appropriate issue. We will be sure to inform you of any developments regarding this feature.
We've fixed it so Document.BuiltInDocumentProperties.NameOfApplication will get to the PDF Application field.
But PDF Producer will always remain Aspose.Words generator name.
This will come out in Aspose.Words 10.2 at end of June.