Ingenious, but I don't think the rivnut will stay put if you need to torque down on the bolt.

Idea:
Although this won't be straight forward due to the chroming on the cap, try and weld a nut to the face of the steel cap. The welds will be stronger than the compression of a rivnut.
Plus the hex allows you to attach it to the arms with a wrench, not a pin tool.

The head of the bolt needs to be 11.42mm in diameter to push against the axle. (I measured the Park puller I have here at work.)