Hi,
Seems a nice plugin.
I tried to Network activate it as described in the Install section.
It was activated but :
- doesn't show up in Site Admin > Tools, but in each sub site > Tools section
- sends a 503 response status for the included plugin stylesheet ;-)
Did you plan on updating the plugin to work on latest WP version?
Thanks
http://wordpress.org/extend/plugins/wpms-site-maintenance-mode/