My group moves a lot of legacy sites into multisite WordPress.  Quite often we will also map a client’s existing domain and set up redirects so established links will resolve.  We really like using the plugin Simple 301 Redirects, but it does not work with domain mapping.  I’ve added a couple of lines of code to the plugin that allows the plugin to function when a particular site is using domain mapping.

The plugin works the same way as the original, so you can redirect requests to .php, .htm or whatever legacy paths you want to redirect.  Download my patched version at git hub. Thanks to Scott Nellé for writing a great, useful plugin.