Using the script at http://www.undefinedfire.com/lab/changing-existing-git-authors/ as a basis, generated the following shell script:

#!/bin/sh
if [ "$GIT_AUTHOR_NAME" = "nuclear_eclipse" ];
then
export GIT_AUTHOR_EMAIL="jreese@leetcode.net"
export GIT_AUTHOR_NAME="John Reese"

....

elif [ "$GIT_AUTHOR_NAME" = "(no author)" ];
then
export GIT_AUTHOR_EMAIL="unknown@users.sourceforge.net"
export GIT_AUTHOR_NAME="Unknown"

else
export GIT_AUTHOR_EMAIL="$GIT_AUTHOR_NAME@users.sourceforge.net"
fi

export GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
export GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME

Then used git-filter-branch to update the entire repository history with new author details. All authors not specifically handled in the script maintained the current author name, but was given a “<username>@users.sourceforge.net” email instead of the default hash assigned by git-svn.

$ git filter-branch --tag-name-filter cat --env-filter '. ~/__git-new-authors-shell' -- --all