The filter on plugins_url, domain_mapping_plugins_uri line 678b at the time of writing this, still breaks the directory path for mu-plugins.
Dave wrote a fix here. Not sure it is the best way to do it, but it'll give you context.
http://davoscript.com/blog/desarrollo/wordpress-mu-domain-mapping-plugin-and-multisite-mu-plugins/
Otherwise, the use of PLUGINDIR is deprecated anyway. Thanks!