I'd even argue that this is a prerequisite to making any visual language workable. No language that doesn't play nice with source control is going to get off the ground. If you also need to come up with a visual merge tool before anyone can use the language for anything complicated then you've got a minimum viable product that's infeasible.
One potential problem with this scheme that then pops to mind after that is the a version situation that led to VB.NET's failure. If you've got two visually different languages that are otherwise functionally equivalent (for the most part), what's the use of learning both? Probably most people will just settle on the one that you can't get by with not knowing - in this case, the textual format - and not bother with the one that's not strictly necessary to know.
Most of these visual programming tools have long been using XML as a storage format, which is essentially equivalent to using S-expressions for it. You could edit the generated XML directly, but it's a horrible experience. It would make no difference to the end user, unless you generated S-expressions that were actual human-readable and editable Lisp code.
Even if XML isn't the cause of all the problems with the format's hand editing experience (it probably isn't), the decision to use XML is a clear indication that human edit/readability was, at best, an afterthought. Probably nobody spent any time worrying about making sure it was a good experience.
This is what I think we really need. A general-purpose programming language with first-class dual mode (visual and textual) editing. A Lisp/Scheme-like language would perhaps be best suited for this, although I've seen specific applications that did it with Java (GUI code generators) or SQL (query generators). It needs to have true two-way functionality so the programmer can switch back and forth at will--the visual editor has to generate code that a human can easily read and edit, and it also needs to be able to generate a readable visual representation for code that was written by a human. And if you make a syntax error while in textual mode, the IDE should warn you proactively that your code doesn't compile, instead of just waiting for you to switch back to graphical mode and breaking horribly, and so on.