Is it much harder to add an additional hole and plate? I think it would be my preferred option. In a crash you would have extra load through the bolt from the harness, but it would probably be fine. I'd be more concerned that if the cage did punch through the floor or something it would take the lap strap with it. I wouldn't want roof crush loads going through my pelvis. Or in a side impact the mounting point might come inboard if the cage moved and loosen your belt. I'm just thinking out loud here, I could be completely wrong! But I imagine the scrutineer would probably rather see it done by the book for the sake of drilling a hole.