AFAIK this is not possible. When you run LoadWB, the "Workbench" and "Windows" patterns are loaded. The only pattern you can see before LoadWB would be the "Backdrop" pattern, which is hidden as soon as you load Workbench.
Edit:
The backdrop pattern is of course only hidden if you run your Workbench as a backdrop window (which I estimate 99% of all Amiga owners do :-) ). If you don't have a backdropped Workbench you will see this pattern behind the Workbench window if you resize it.
