I appreciate your "sharing", Smince. I think this is a good example of how someone can get involved in an encounter outside the "dark alley" or classic "home intruder" scenarios, without having the "Mall ninja" looking for trouble mindset. Your situation is very easy for many of us to relate to.
In a rural area with a dearth of law enforcement, I think there is some grayer areas with regards to calling 911 then in a more urban area. In principal, I'll concur with the others here who advocate calling 911.
But I do think there can be mitigating circumstances. Considering that you could be taking the only LEO for miles around out of position for a more urgent, life-threatening call for possibly nothing would give many pause, including me. I would not condemn as a vigilante anyone who pushed it a little further to see if the law was truly being broken. I might, depending on the complete circumstances, actually start to clear the building, but would very agressively break contact and withdraw at the first sign of current occupation.
It all depends though. I do think it is sound civic-mindedness to consider the the "thin blue line" in ones equation. I can understand your sense of impatience, and potential embarassment. May I ask generally where you are experiencing your illegal house-squatting problems? From poultry and textiles, I'd be guessing Carolina?