I looked at this at one point too. I think the hbox is transparent anyway. The white background isn't due to the hbox, but rather the ancestor widget (I can't recall if it is a window or a special hildon widget in the case of home applets).

In case you haven't viewed this example (posted on a similar thread in these fora), you can look at:

http://wnd.katei.fi/maemo/desktop-plugin-example/

I think my stumbling block to implementing this was the fact that I haven't dealt with C++ in such a long time. I prefer to work with plain ol' C at this point. Hope you have better luck!