http://www.youtube.com/watch?v=A-hHwQ32UoE
Actually, they used a Boba Fett figure, (large and small) for all of it. Boba was standing in for the Biker Scout, as an action figure of it didn't exist yet, and was the one character similar enough in design to the scouts.
If Grevious wasn't supposed to be Maul, it should have been a simple matter to scan some of the preproduction art and use the eyes. Using Maul's eyes was a deliberate choice. Whether it was it was a subtle ploy to sell George on the idea, or an abandoned concept is up in the air.