Yes it's possible.
I was in sunny Los Santos yesterday (not watching the Superbowl), and I was at the Mile High building in passive mode standing on a pile of building materials. This griefer in his stolen Merryweather Mesa starts honking at me, shooting at me with a pistol, but all in all, just being a bored free roamer, somewhat harmless once you know how to use passive mode.
So I start walking into the middle of the building in passive, and the guy is following me, not noticing or not caring that the "gang attack" red circle has appeared. I walk right into the middle of the circle, then I run for cover behind a column, instantly 15 gang bangers show up and start firing and taking position. The griefer behind me gets picked off, and he's dead. Now I'm in passive (or so I thought, more on that later), and I can't pull out a weapon. So I'm not sure what to do, except what I would do in real life, call 911. So I call emergency services and ask for police. In about 10 seconds, a black and white with 2 of Los Santos finest pulls up, and with guns drawn they are having a raging fire fight with the 15 gang bangers. The griefer comes back, and he's in the mix. It's an all out fire fight, and I'm still ducking behind a column watching it all. Gang banger falls down, griefer gets shot, another police car shows up, it's getting heavy.
Finally, it's down to the last gang banger, and he gets put out of his misery by the original griefer. Suddenly the gang attack success screens pops up. I got 600 RP. Afterwards, I went to the some of the corpses, and picked up about $1500 in cash, some of them were good for $500 alone. All of this for just watching the show.
So I'm about to walk out, and then the griefer decides to pot shot me by trying to punch me, and surprisingly the punch lands. I guess the gang attack must have put me out of passive mode the whole time anyway, so I engage in a fist fight with the griefer and he knocks me out after I get a few hits in. I immediately go back into passive mode.