Can't. Like I said, it's not supported by the OS, but for a different reason than the one mentioned in the diagnostic message.

It would be possible to do using a temporary file or by slurping the file into memory. This former would also allow undoing the edit in case of error. This has been proposed on the p5p and has gained some traction. The discussion was tracked in RT 1154

Update: Oh! I just realized you meant patching the message.