If a browser supported PUTting the modified resource back up to the server, we'd be off to a good start. Server-side frameworks could then add functionality to split the modified resource back into the files that created it.
Instead, why not make something like IntelliJ/RubyMine/WebStorm/etc.'s Live Edit plugin a native part of the browser. Allow the asset pipeline to tell the browser it has changes available, rather than try to have it regurgitate all the processed assets. That would be relatively easy to add and very straightforward to implement.
The simplest middleware that would pop up would be a diff generator. But with source maps, web frameworks could track what exact source files were used.
That would be really something.