I've noticed that some system directories, when dragged to the Finder's sidebar, utilize special small-scale icons not visible in any other place. Even when looking at one of these folders in a Finder window using the smallest possible icon size, these "special" icons don't appear (so it's not just the small version of the folder's icon).

So my question is, where is this information stored? If I wanted to duplicate this behavior for an arbitrary folder, where would I need to look?

I like to replace my home directory with a symlink to a location on another partition, but when I do this, I lose this sidebar icon behavior. I would love to get this back if I can.