As a result of T59867 I decided that it would be best to allow images to be flipped via the user interface (and thus python). Thus, image flipping has been added to all file formats in the ImBuf loaders. Suitable flags for ImBuf and Image have been created, with RNA properties for the new Image flags and toggles in the image template.
Please use the "Submit Code" button on the main page to submit patches: https://developer.blender.org/differential/diff/create/
Feel free to add me as an initial reviewer, I'll add other reviewers if necessary.