How to index to a particular commit without touching HEAD or any other ref?

Question

Basically, I'm looking for the opposite of git reset --soft - ideally a plumbing command.

I need to modify a particular index file (set via GIT_INDEX_FILE) to point to a specific commit, without altering any ref like HEAD (or anything else).

Is there a command that can modify an index in this manner? How can I achieve this?