I can tell you that the small mangosteen plants are extremely sensitive, even to mildly cold weather, but also other things. I wouldn't try permanently planting it in Orange County unless you live in the more temperature moderate part, and have a good microclimate and spot in the yard for it, where it will be surrounded by other plants, partially shaded, will mostly only get morning sun, will get more sun into the Winter when the sun is at an angle, protected from wind, etc. In other words all the conditions would have to be completely optimal. I believe it's theoretically possible but you'd really have to know what you're doing, and have the right spot for it.
Do not put the seedling in the ground!
Nurture it and wait till it's at least 2 feet tall until you even think about leaving it out. And even under optimal conditions it may be quite some time before it reaches 2 feet tall, they've been extremely slow growing for me, and I have the temperature and humidity set at a perfect level.
I was told by the owner of ONG nursery that he knows of some Vietnamese people in Hacienda (out in San Diego, neighborhood close to the beach) that are growing Purple Mangosteen outside in a container, leaving it outside during the Winter under their patio, and that they got at least one fruit. That's what he told me, as best I can remember. That area is bordering on the edge between zone 10 and zone 11 so don't automatically think that success will transfer over to where you are.
Also realize the West Coast has had a very mild Winter this year.