Also, from my streaming today, you can easily cover your hand. The biggest problem would be actually the drawing animation.
Ways of getting around this would just be putting a stream delay. I set mine at 90s, so with the additional 20s on my stream, that's effectively 2min that they'd have to account for.
Aslo, yeah, honor system. If I were streaming for an event here, I'd trust that the opposing player wouldn't be ghosting my stream just to get the upper hand. Or have an associate do it for them and feed them the information.