Here is my setup: a working WP multisite network (with subdomains) running WP 3.5.1 and last version of domain mapping plugin.
To clean the code and hide that WP is being used, I have the following line in my wp-config.php :
define( 'WP_PLUGIN_URL', '/plugins');
When I network-activate the domain mapping plugin, a lot of URL are messed up (ex: http://www.exemple.comr/image.png). After looking in the code of the plugin, the error comes from line 651, in the domain_mapping_plugins_uri() function.
Could you check against absolute plugin URL before returning the mapped URI?
Is there a workaround to make domain mapping works alongs with custom plugin path?