Enable external image processing in compositor by extending image input and output nodes (e.g. GIMP)

I'd like to script an external program like GIMP to supplement the compositor. In Blender 2.8+ there is an image input and image output node. As implemented today, these nodes only work with a single image. If these nodes supported image sequences and the image input node blocked compositor execution until a new image was available, then it would be very straight forward to add new compositor features external to Blender.

Example use case: A compositor node network is created that includes 'Image Sequence Output' and 'Image Sequence Input'. The image sequence input node prevents the compositor from continuing until a file is available. An external script create a new image after processing.

For each frame, the following events occur:

1. An image is exported from the image sequence node with a new name (the name is part of an image sequence like img01.png, img02.png, ... ).
2. The image input node waits for a file with correct name in the image sequence to appear. For example if it is frame 1, the node waits for 'new_img01.png' to appear in the directory.
3. An external script can be written that monitors the directory and when it sees a new file (img01.png) appear, it processes the image and generates a new one (new_img01.png).
4. Once the new file is available, the image sequence node loads the image and passes it along for continued compositor processing.