That's only meant for xecuter loader, for multigames xci files so you can set something like a collection icon. Like the one I did for Metal Slug multi xci.
It just takes a control nca from a nsp or nca to put it in the secure partition an it doesn't modified the one loaded in Horizon. I could add something to patch icon, name, editor,... in a nsp or xci directly in the future thoug. If i'm not mistaken the icon should be a 256x256 bitmap and I already now where the name, editor, ... is located. Is a matter of replacing those byes directly and rebuild hashes.
Though other thinks take priority currently. For now you can use hacpack from the4n or xortroll's gui if you want to change icons, though it won't be as automated...
Edit:
Actually I see the merit in patching directly the nca files in the case you're talking about, so you can conserve easily the flags that it can have in the ncap file.