Yes, you can use the plug-in in offline mode. I've checked the description and details on the official website, but there is no indication related to the Internet requirement for the software.
You can download the latest version from the following website: https://www.lizardtech.com/gis-tools/plug-ins
These are the requirements:
Microsoft Windows XP, 2003 SP2, Vista SP2, 7, 8, Server 2012
200MHz PC processor
Minimum of 64 MB RAM
1GHz processor
Firefox 25 or later
Internet Explorer 10 or later
Not supported on Chrome due to lack of NPAPI support
Not supported on mobile devices (see GeoViewer for iOS and Android support)
