Sounds like fun! I guess there is a trail that says you need 4x4 to get out to south point and I think there is another beach beyond that where all the plastic crap washes up from the sea. Where you located?
I'm just south of Kona. I've been to South Point, pretty fun. Never been past Green Sands though. I found "Big Island Jeep Club" located out of Kona on Facebook, you should look them up. Seem like a good bunch.