Somebody at work just stepped a Microsoft Dynamics "integration". The network admin is trying to recover the integration database from a snapshot. An integration maps data from external files into accounting structures: it uses a GUI, with names that make sense to accountants. And the database that holds it is an Access database with the integration details in a BLOB. I'd pay money for a tool to save these out to a textual representation we could put under source control.

There's no reason a diagram can't be in source control. You just need a tool that can convert the diagram to code and the code to a diagram. You probably also want to store the diagram's layout in textual form too so it can be converted back to the original human-created layout.

Then you can use git and optionally build custom tools like a visual diff of diagram v1 vs v2.

The diff tool is the key here. Without that, there is no meaningful version control.

With a visual representation, diffing may be as easy as clicking next/previous between versions as you would images in an album. You could of course add more bells and whistles with highlighting.

Sorry, I should have been more specific. When I say diffing tool, I mean one that can do what diff does - help Git do automatic merges, and help me do manual merges when auto merge finds conflicts. Two pictures side by side can do neither.

I don't see why it couldn't be built. Git merge doesn't always do all the work for you, you still need to evaluate the changes.