While Just Cause isn’t really on my radar, the second one in the series had a relatively popular multiplayer mod that released a few years down the road. According to PC Gamer, there was another one in development, this time for Just Cause 3, but it was canceled as the developer was hired by Avalanche Studios.
At the same time, a second mod team, called nanos GbR, has released their own version of Just Cause 3 multiplayer. Why? Just ‘cause! Also, they are promoting their multiplayer framework, which includes plug-ins for Unreal Engine and CryEngine. It sounds like they want to be integrated into commercial projects, but, try as I might, I can’t seem to find anything concrete by them besides their mods to existing video games.
The Nanos Just Cause 3 Multiplayer mod is in public beta and available for download.