Despite @JohanLarsson providing the correct answer for 2012+, I've opted for the .
prefix on the directory:
:
+- ClassC.cs
+- ClassD.cs
+- .Metadata/
| +- ClassA.Metadata.cs
| +- ClassB.Metadata.cs
: :
This renders the namespace name provided to classes created in that folder invalid. It's more of an implicit indicator than an explicit rule as I'd have preferred (and as @JohanLarsson's answer provides) but then again, I'm the sucker running 2010.
I'm still open to answers, and will gladly change acceptance to something better.