Should work then.
I was writing out you could probably jamb nut a bolt on there and use a box and wrench, then I realized most often 1/4 hardware is 7/16...
For some reason, coupling nuts vary somewhat, so you might need to poke around a little to find a different brand with the external hex you want. You could also, of course, weld together a few nuts of the appropriate size. Clean up the exterior of spatter, and perhaps drill out the center a little more to suit the pin if needed.
Motorcycles often have a large internal hex in the front axle (12, 14, 17, or 19mm) and the lightest, cheapest, and most compact option is to carry a metric coupling nut of the appropriate external size and use a wrench. You can also create tools by welding up a bolt and a nut or two, or lopping off part of a hex key or hex stock, etc.
Could you simply defeat the security feature by grinding off the post in the female hex opening on the bike rack?
Grinding with what? It's pretty much unpossible to get very far with a Dremel bit without absolutely mangling the sides of the hex.
Some have found success with similar fasteners by using a small chisel to break the pin off with a sharp sidewards strike, but this fastener had an unusually large and strong pin. And if you only bend it instead of breaking it off, you now have an even worse problem.
A teensy tiny Dremel bit and an unusually steady hand might be enough to create a dimple on the top of the pin as a starting point for drilling the pin, also using an unusually steady hand, and the very finest quality small drill bits. This would also be quite difficult, since you can't get the whole rack or vehicle into a mill or drill press, and the tops of these pins are usually somewhat rounded.
