There is no public property where it is legal to "offroad" or operate a non-street legal vehicle on Orange County. There was a private off-road park by Irvine lake, if it's still open that would be your best local option.
+1 on the Trabuco/ Coto-ish area. These girls actually took me there, one area is called Holy Jim canyon? A few weeks ago I took my ranger back there just to mess around with the HIDs. Loud supertrapp may be a no-no but worth a little try its a decent powerline-like road.
There are many hikers, bikers and horse riders in trabuco cyn. I would not recommend using that for a test location. Why not give Bob a call at Glen Helen? It would be a hell of a lot safer and more worthwhile.
Contact: Bob Beyer, Glen Helen Raceway – Off-Road Director
Phone: (909) 815-5811, Email: email@example.com