You could certainly do it though a migration, and I've seen that as a recommended way to add necessary seed data to an application's database. Migrations are just ruby code, so it's quite simple. You can use all the ActiveRecord stuff you're used to and do something like:
class AddRootUser < ActiveRecord::Migration
  def self.up
    user = User.create!( :email => '...', :login => 'root', :password => '...' )
  end
  def self.down
    user = User.find_by_login( 'root' )
    user.destroy
  end
end
I've left out things you'd probably want to do like rescuing a failure to create the user, etc. Also, I haven't used AuthLogic so I don't know exactly how its user model works, but you should be able to figure it out from here.
Another common pattern is to have the down migration completely clear the table and have the up migration call the down migration before running. That's probably not what you want in this case, though.