You're right. That is pretty extreme. I'm basing it off of the figures at: http://www.edmundoptics.com/products...productid=2482 ,
and http://www.edmundoptics.com/products...productid=1942. Depth of field numbers are always a bit subjective, as they rely on assumptions about acceptable circles of confusion. I have one of the Mitutoyo 2x objectives, and so I'll be able to test it. It's not like the figures I listed are a cliff and quality drops like a stone if you go outside of it. Rather, you'll start to lose detail in the areas where the height variation increases. As you say, it's very doubtful that most commercial scanners have tolerances that tight.
You can certainly mount a photo (or something) to a very flat piece of aluminum, but I've been mounting film directly to a thick sheet of glass.