Hi there. So, a couple of things to think about (and some questions):
1- Do you know where this plug-in gets installed on the system? There are 2 "Internet Plug-Ins" folders. One at the root /Library level and another within each users home directory Library folder. Sounds like maybe its being installed to the user level Library/Internet Plug-ins/ directory. Yes?
2- If the above is true, have you tried placing/moving this plug-in into the root /Library/Internet Plug-Ins/ folder and seeing if it works for all users at that point? If so, just build a package with it being deployed there and you're done.
3- If it doesn't work placed there, capture it in any user level Library/Internet Plug-Ins folder, as long as the home directory is in the /Users/ path, not someplace like /private/var or such. Package that in Composer as a DMG and when uploaded into Casper Admin, use the FEU (Fill Existing Users) and FUT (Fill User Template) options.
Alternatively, package it as a .pkg dropping it into someplace like /private/tmp/ but include a postinstall script that loops through home directories and copies it from /tmp/ into their /Users/<user>/Library/Internet Plug-Ins/ folder. Search here for example scripts that can do that. There are several.
