Thought this was interesting...
I put dropped the whole eventcalendar3 folder into the plugins directory, and with Site Admin->Options->Plugins Enabled, I am able to Activate and Deactivate the plugin individually as required on each blog.
Now I understand that if you put a plugin in the mu-plugins directory, it will auto-activate and you will not see the PLUGINS menu appear on the admin screen. BUT... if you drop the whole eventcalendar3 folder into the mu-plugins directory, it does not recognize the plugin at all. You need to take everything out of the eventcalendar3 folder, and put it directly into the mu-plugins directory, and then it will work.
The only problem with this, is that if you need to remove a plugin, you now have your mu-plugins directory full of a bunch of files, and you don't know which ones go with each plugin.