difficult spot for sure! I have a couple ideas that might work...

1. use a SHCS (allen key screw) and put the bolt up, then tighten the nut. I don't love this idea because the bolt is down. Just against good practice for me...
2. jam a cold chisel in there by the nut and tighten the bolt. If you use a chisel at least it is tapered to allow a drift to knock it back out.