Can't resolve custom icons in the panel

[Tim Cuthbertson <tim gfxmonk net>]

I'm still trying to get custom icons to appear in my extension's panel
indicator. I've done the following:

 - added my icon to
$XDG_DATA_DIRS/hicolor/48x48/actions/window-tile-horizontal.png
 - created an StIcon, and added it to my indicator's panel actor

If I use icon.set_name("go-down"), the down arrow appears in my
indicator. So it seems I'm hooking up the icon / layout correctly.
However, if I use icon.set_name("window-tile-horizontal"), no icon
appears.
I am sure that the icon is in the right place, as:

$ notify-send --icon=window-tile-horizontal "test"

works and correctly shows my custom icon with the notification. Is
there some difference between how notify-send and StIcon resolve their
icon images?