It is also worth noting that Campagnolo chainrings are not of common thickness, so there can be an issue with fitting chainrings not specified for a given crank set - the "hidden" bolt means that the chainring has to have the appropriate thickness for the cranks and spacer that you are aiming to attach it to.
The offset bolt is there so that users can't make a mistake with regard to the relative positioning of the two rings, in order that the ramps and pins and tooth profiles are correctly placed to lift & drop the chain.
Thanks for that.
Looking at my chainsets though it appears that differences in thickness of the chainrings is more of an issue with the outer rather than the inner ring. A slightly thicker inner ring should still sit at the same distance from the outer ring at the offset point and the same spacer could be used (and bolt should still be long enough to be tightened). However, a thicker or thinner outer ring (which attaches directly to the crank arm) would cause problems as the distance between it and the inner ring would therefore vary.