Magnus' commons Helpter tool currently solves this by providing a link which leads to the original file in edit mode with the template parameters inserted already:

This example shows how it would look if the file name on Commons was changed. If no change occured the source code would simply say {{NowCommons}}

Two quick mocks:

What we could show if we can link directly to the edit page of the original file like the Commons Helper does. We should think about if there's a way for us to provide a link back to Commons after inserting the template.

Should the automatic insertion not be possible we would just provide the code to copy and insert manually. If the filename was changed the text that needs to be copied should be changed as well. This is not necessary in the first mock I think.

…one could also think about doing this more notification (so non-blocking) than modal style (depending on how essential this is)