Ok, I FINALLY talked to someone who knows if that trail is legal or not.
According to Greg, the guy I talked to at Draper city (or was it Gary?). I think it was Greg. Anyway, he informed me that Zion's Bank owns the land just north of those access roads (lighthouse and hog hollow) in highland and alpine, and that ALL MOTORIZED VEHICLES ARE PROHIBITED there. Even though there isn't a single sign.
I told him that we would be happy to lend a hand to post signs and inform people, and he appreciated it, but their signs require a backhoe, cement foundations, etc. So he appreciated our offer to help, but he said that it was in in-house job. He was very nice, and very helpful. I'm sad to find that that area is off-limits to motorized vehicles (but for some reason it's ok for mtn bikes, hiking, etc), but now I know.
So a HUGE thanks to those who urged me to check, double-check, and triple-check the legality of that wheeling area.
That being said, this noob run is still on, but we will be changing the location.