A picture would be worth a lot more than my attempt to explain here with words, but the Simplex hanger attachment system differs from Campy in that there is a threaded bolt (not sure of the threading, but it's significantly smaller than M10) that threads into the upper pivot of the RD from behind. The bolt has a non-threaded step-up (for lack of the proper term?) between the head and the threads which is wider than the threads (but still slightly narrower than 10mm), and when installed, this segment of the bolt is situated inside the non-threaded bore of the hanger, like a bushing. The step-up seats the bolt fully against the pivot body when tightened and is just slightly longer than the hanger is thick, such that the bolt can't compress the hanger between the bolt head and the locator plate, so the pivot can rotate freely. It's basically the reverse of a Campy arrangement. And yes, a bolt with a longer step-up segment would have to be machined to fit properly in a Campy hanger, otherwise the RD wouldn't be able to pivot freely with the bolt tightened. Making a bolt that would work universally would also assume that all campy hangers have the same thickness, which I don't think they do? Because differences in Campy hanger thickness are inconsequential to mounting a Campy unit to a Campy hanger.

You'll totally get it with one photo. I'll try to take a picture tonight.

EDIT: On second thought, it might be easier to just machine a bushing (or find an appropriately sized one) that's just slightly longer than the thickness of the hanger, and use a generic shallow button-head bolt in place of a proprietary bolt with step-up. I'm thinking something like a jockey wheel bushing would be a starting point...