Here is a bruder conversion of a mower with two blades. The end of the video shows a quick photo of the underside of the mower deck.
http://www.youtube.com/watch?v=tRLMXiS0g5Y
It looks like a scale mower has the same problem that others have with their scale diggers, and that is that the grass you are cutting is still full scale just like the dirt and rocks don't scale down. I am just guessing here, but something like a scale bush hog may be the most feasible mower to build that may also be reasonably functional. Bush hogs are made to cut through heavier grass and brush, and the grass looks like it scales more like brush. You would need lower RPM's than a finish mower, but higher torque. May be a bit safer.