Thanks for the reply Louis Moreaux ! I forgot about that function. But in the meantime, I've solved it using a custom AJAX callback with APEX_DATA_EXPORT.DOWNLOAD that get's called via APEX_PAGE.GET_URL(P_REQUEST => 'APPLICATION_PROCESS=... syntax from my SQL query. Works like a charm :)

Amazing post, thanks for that! Do you have any tips/pointers on how to realize this with images stored in BLOB columns? Besides using a RESTful services that delivers the images as a media resource :)