Currently if the author of a commit is not in the authors mapping their SVN username is recorded as the change author - if that author is added to the mapping e.g.: later through a hg -A authors.txt pull their existing commits in the hg repository remain as commits with their SVN username. This occurs frequently in my case as I am working with several large, distributed, teams on a number of projects and am often only aware that a new developer has joined a project when I start noticing commits from them, (with their SVN username).
While it is possible that hg rebuildmata would correct this, since it uses the remote repository extensively, it is likely to be very slow for large repositories or when there is limited bandwidth.
Since the data is all stored locally, i.e. both the commits and the new author list it should be possible to correct the usernames to match a new or updated author list - this could also be very useful when an existing developer gets a new email address, this often happens in our organisation, especially when people change their name, e.g. on marriage.