Your hitch looks different than mine. On my hitch there are two carriage bolts on each side and a long piece of metal on the hitch to connect them to the frame. But your hitch doesn't appear to have that piece of metal. That aside, the access hole should be big enough to shimmy the head of the screw and that small plate up through the hole.
So..
1) put plate or small piece of steel about 1/16 -1/8 in thick on bolt
2) take a small spring, attach a piece of wire to it, attach to end of bolt, shimmy through hole and let hang. Or find an access hole by dropping the frame a bit. Run wire through and pull.
You can use any spring and wire or you can put some double sided tape around the end of the screw, rap wire around tight and put some electrical tape over the top.
Hope this helps