To be clear: gofmt itself should not contain the actual -ss code - that code should be in an independent package that can be invoked from elsewhere and that's simply hooked up to gofmt.

Make a separate tool. Let's not keep piling things into gofmt just because it happens to read and write Go files.

I'd be happy if you want to put this in grind.

Separate tool is fine. I'd still suggest to have the actual machinery factored out into a library so that it can be used easily elsewhere.