There might actually be nothing sinister going on here; there's no obvious copy protection, or anything like that. They just couldn't do what they wanted without breaking the redbook. Here's the relevant chunk of the DualDisc Physical Specification:
PH 1.0 - Overall DualDisc Dimensions
Overall thickness and diameter dimensions of the DualDisc will conform to the DVD Physical Specification, dated May 2005 or later (maximum thickness = 1.50mm).
PH 3.0 - CD Media Side
The CD media side of the DualDisc minimum thickness = 0.870mm
Of course, you can't really get a copy of the DVD Physical Specification unless you've got lots of money to burn, but that's not important. You can't get the Red Book either, but you can get IEC 908 for quite a bit cheaper if you try. Or you can photocopy it at the library. Now, IEC 908 (aka IEC 60908, for reasons that I'm not clear on) doesn't specify anything called "thickness", as was specified above. It does specify: "6.1: Thickness of transparent substrate: 1.2 +- 0.1 mm".
From quite a bit of googling, this appears to be the root of the problem. It's also why some drives read them just fine, and why it varies from disc to disc. More later when I have time to experiment with a drive that can't read any of the DualDiscs I've got.