If I'm not mistaken, you'd have to show that the HOA was liable in some manner. If he was "well briefed on neighborhood watch guidelines", then the HOA did what they were supposed to do, and Zimmerman acted on his own. To put it another way...
If Zimmerman crashed into another car on common HOA grounds (while "patrolling" as he was told not to do), would Zimmerman be solely liable, or would the victim be able to sue the HOA for the incident Zimmerman was liable for?
If believe he didn't sign something exonerating them of blame, then they are still liable, especially since this happened in a commons area where people are allowed to walk around.
Zimmerman's idea of "patrolling" and Neighborhood Watch's definition are probably two different things.
The woman who set up the program for that neighborhood told them not to leave their homes or vehicles for instance.
From the New York Times
SANFORD, Fla. Last August, Wendy Dorival got a call about setting up a local neighborhood watch. As the volunteer coordinator for the Police Department here, she gets such calls regularly, and the city already had at least 10 active watch groups. So she thought nothing of this call, from George Zimmerman.
She set up a visit for the next month at the Retreat at Twin Lakes, a gated community that had been dealing with a string of burglaries. When 25 residents showed up, a decent turnout, she had the residents introduce themselves; after all, people join the groups to look out for each other. She then gave a PowerPoint presentation and distributed a handbook. As she always does, she emphasized what a neighborhood watch is and what it is not.
In every presentation, I go through what the rules and responsibilities are, she said Thursday. The volunteers role, she said, is being the eyes and ears for the police, not the vigilante. Members of a neighborhood watch are not supposed to confront anyone, she said. We get paid to get into harms way. You dont do that. You just call them from the safety of your home or your vehicle.
Using a gun in the neighborhood watch role would be out of the question, she said in an interview.