Love this idea, it would make source modification a piece of cake for every browser-maker to implement. To allow for compatibility, this feature could be activated only when a X-Source-Modification-Enabled (or something) header is present on the original page. The header's value could even be the (relative) endpoint to call!
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.