Brass is less than 1 point harder on the scale than gold for nibs.
I think you'd have to really work at it to mess up a gold nib with brass.
I'd imagine that even brass would be enough to score the sides of the slit if you were to try wedging the slit open in this manner.
Just some belated metallurgucal reflections on this thread.
When it comes to hardness in non-ferrous metals (such as copper, silver, gold, etc.), don't forget that the nature of the metals means that there is great variation in hardness, depending on what has been done to the metal. This quality is universal to all of the copper alloys, such as brass and bronze, and is considered one of their great virtues.
More simply put, you can soften brass completely by heating (annealing) it to a glowing red (about 1000º F), then quenching it. Then, as it is worked, it gradually hardens, to the point where it is hard enough to be used as a knife blade (not a great one, but there is a reason they call it "the bronze age.") The same piece of brass which might be safely rubbed against a gold nib when it is soft, would damage the same nib after it has been work-hardened. Why? Because it started out relatively softer than the gold, but ended up relatively harder.
While there is empirical data rating the hardness of metals, it is not safe to use it out of it's proper context; specifically, the fact that the samples began at the same relative hardness. In practical, pen-related terms, this is important when talking about the interaction between two metals, both of which are variable. This is especially true when you consider that the differences in manufacturing processes--even within one company, between batches--can result in wide variations in nib material hardness as well. (Hence, as a speculative example, the variation in flexibility among identically shaped Parker Duofold nibs. Same materials, same shape, same tools and equipment. The variable? How hard the batch of gold sheet was when they started.)
When it comes to working on nibs, the thickness of the materials involved also comes into play. Brass foil sold as 'nib floss' is thin enough that it cannot transfer the force required to scratch the quite hard gold alloy of a nib: rather, it crumples and absorbs the force itself. A similarly thin piece of steel (like a razor blade, although to be fair that is still much thicker) is much stiffer, as well as harder.
So, in short: brass foil is safe to use on nibs, steel (of any kind) is not. And: if you don't know what you are doing, you might well wreck it (and also make it ugly.)
It all makes one wonder just what Dr. F could accomplish given a Dremel tool...