I don't know of any service that does this, but you could write a process that could analyze every line in a given repo. Weight for each line of code would have to be balanced against the number of lines previously written by that developer, as well as the date of the commit (more-recent commits weigh more) and perhaps the TFS work item associated with the changeset, and other details of the TFS work item/bug.

IMO, it seems a lot more work than the value can provide - a simple answer from one of the devs suffices, it's what the process has been for me.