I don't like the idea of using an OCB buckle like that at all.
@DanO could weigh in on what they're rated for, but it's a far cry from what climbing rope and biners are rated for. I wouldn't trust those rivets to hold me if something gave. Hundreds of pounds vs thousands.
The shackle is probably fine. I had a screw lock one that was good for like 4,000lbs. They usually say on them what they're rated for.
I'm toying with the idea of using a riggers belt as a hookup. Something like an austrialpin cobra with the d ring. Clip it around the tree, and then run your bridge through the d ring. The materials are rated for the application, and you're not really using it for anything wildly different than its intended purpose, but I feel hesitant recommending it because I haven't played with it. I know you'd be SOL if you managed to bump the cobra buckle loose, but that might only be a concern on paper. I don't know. Haven't played with it yet.