I'm not so sure that would be convincing. Why would God appear in one body rather than another? How could we tell the difference between a deity and a superintelligent and powerful extraterrestrial organism?

For a theophany to work, the being would have to be self-evidently divine, leaving no room for doubt. In other words, the encounter would have to be miraculous rather than just empirical. A guy walking around wouldn't be miraculous. Even if he walked in an amazing way, it could be due to advanced technology that only seems miraculous. A real miracle would have to be self-authenticating. As to how an event could prove itself in that way, I have no idea.

A psychedelic experience comes close since it lowers the person's inhibitions. But as soon as it's over, doubt can set in. Perhaps if the miracle never wore off, so the experience with God would act like a trauma that transforms the person's mind and character, doubt would never set it. But a lobotomy could work just as well, as could a convincing illusion.