Export layout customization will be done through project-specific plugins which can cater to their individual export image requirements. An export plugin is expected to take over the mid-point of the image generation process during export.
Unlike table plugins, an export plugin does not have any UI component (except for possible options added to the export settings panel) and reuses the default export dialogue. Also, an export plugin is not involved in the last stage of merging separate images into a single export file (this includes checking for tainted canvases according to the config options and providing feedback to the user if the image cannot be saved automatically).
This is intended to simplify development of export plugins, since they don't need to generate their own UI (which generally should stay generic) and deal with tainted images. This, however, does not preclude a plugin develop from disabling the default export functionality and writing a full-replacement export plugin with its