I would like to propose an addon of mine for trunk integration if it's not too late.
It is called Auto Reload Images.
It's a quite simple addon which is designed to help with reloading images datablocks.
It has 2 main operators, one is a "Reload Modified Images" which will check for modified images and reload them, and the other one is a modal timer, which check every n seconds for modified images.
If it has to, the viewport rendering will be retriggered to update it accordingly.
Buttons are located in the scene tab of property area.
The modal operator also has a draw function in this same area to warn the user it is still checking for modification.
The usual way to use it is if you are working on a texture (or set of textures) in Gimp, Substance, Krita... you just save it and you can see it in action right away in EEVEE or Cycles.
The logic behind it is to attached modification date property to image datablock and check for it on demand. There is also a startup handler to update modification date.
This addon is pretty simple, but it could be useful in shading and texturing situation.
A video overview of the previous version (on 2.7x sorry about that, didn't updated it)
Thanks and have a good day !