We've been discussing about the possible removal of the external image editor feature with some Construct Community members. Since you guys know all the details on what is and isn't possible, I'd like to propose the following system as a replacement.
Feature proposal: Local Image Sync
Replace the "external image editor" button with a "local image sync" button. If this button is pressed, Construct 3 will sync the current frame with a selected source file. If any synced source image has been modified by the user, Construct 3 could open the sprite editor to confirm/apply the changes.
I'd imagine reading the state of a file is possible using the new native file system API.