update index mtime etc metadata

Is there any available plumbing that can change the mtime etc metadata that is recorded in the index for a file, to user-provided values? Or, to force the current file stat metadata to be updated in the index?

I know, git update-index --refresh, but I have a case where that's too expensive. I'm using smudge filters; I know that the cleaned version of the file will be unchanged from what's in the index now and only the stat metadata will change, and so I want to avoid git update-index --refresh running the clean filter, which can be quite expensive for a large file.

At the moment I don't see a way to do it other than using eg libgit2 to update the appropriate fields in the index structure.