Alterego said that it is pretty straightforward, so I think you should see his result in a few days. Although it is a nice exercise if you want to deep dive into hildon desktop's workigs.

However, I wouldn't recommend it.
This stuff depends very much on undocumented behaviour of undocumented features of Hildon. So much so, that it required me several days to hack around it. The basic functionality is easy to achieve, but there are corner cases that just blows up your head as you try to solve them.

My favourite:
Usually when swiping to another desktop, Hildon changes the gconf entry (calls the callback), and then updates the widgets' _HILDON_APPLET_ON_CURRENT_DESKTOP property.
However, when the desktop is in edit mode and the user drags the widget to another homescreen after dragging it on the current one, the order is reversed: it sets the property first, and then changes gconf. This caused me headaches for some time. (filed a bug for that one here)