A webclip widget is just an extension of the Webclip widget available in the widget bar when in dashboard. If they were widgets in their own right, they would be available in the widget bar alongside other widgets (so you could close them and re-open them) and would have their own files in the finder.
I already know how to place widgets on the desktop.
The reason I'm wanting to do this is so that I can make a webclip widget that I can close and re-open at a later date without having to re-create it and so that I can send it to other people.