I finally got a nice volume icon working for my Air:
Since this was slightly convoluted, I thought I'd do a quick write-up.
The icon itself is
/System/Library/CoreServices/CoreTypes.bundle/ Contents/Resources/com.apple.macbookair.icns (discovered here).
Assigning icons can be tricky in OS X, as I described in an earlier post. I had no luck with icns2icon this time, even when I tried very liberal permissions, so I went with the kludge described in that post (sticking the icon in an app bundle) instead.
I was then able to copy-and-paste the icon as the icon of the
icns file itself, but for some reason I was still unable to make it the icon of my system volume.
Here is the icon file with itself as its icon, to save you some work. The download is an archive since the icon wouldn't stick otherwise.
I ended up having to use CandyBar (which has a free trial). In CandyBar, I just dragged the fixed icon file to the volume.