C# – BadImageFormatException when AnyCPU test assembly implements interface from x64 production assembly

I seem to have hit on a scenario where when I run mstest on an AnyCPU assembly which references an x64 assembly, I get a BadImageFormatException.

The issue occurs when an interface in x64Production.dll is implemented (even if unused) by the AnyCPUTestingx64Production.dll test assembly:

Unable to load the test container 'D:\AnyCPUTestingx64Production.dll' 
or one of its dependencies. error details:
System.BadImageFormatException: 
    Could not load file or assembly 'x64Production, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. An attempt was made to load a program with an incorrect format.

• mstest is running on windows 7 64-bit
• the test assembly is built as AnyCPU to get it to run in 64bit on a 64bit host (as outlined here)
• the testsettings file specifies <Execution hostProcessPlatform="MSIL"/>
• peverify and corflags reveal nothing interesting
• this is readily reproducible in a toy solution, i.e. where
  • x64Production
    • references no other assemblies
    • includes only an empty public interface IExampleInterface
    • has <PlatformTarget> set to x64
  • AnyCPUTestingx64Production
    • references only x64Production.dll (i.e. this issue is present even without a reference to Microsoft.VisualStudio.QualityTools.UnitTestFramework)
    • includes only an empty implementation of x64Production.IExampleInterface
    • has <PlatformTarget> set to x64
• nunit can load and run the test assembly (once I've converted all the test attributes)
  • but is not a good short term solution to the larger problem (which involves a huge number of project files)
• the same issue arises whether the projects target 3.5 or 4.0
• the same issues arises whether the VS2008 or VS2010 c# compiler is used
• the same issue arises whether mstest from VS2010 or Test Agents is used
• it is mstest which fails while loading AnyCPUTestingx64Production – i.e this is not an issue with attempting to load the assembly in the wrong QTAgent (nothing shows up in Process Monitor and renaming QTAgent32.exe has no effect):

    *** Assembly Binder Log Entry  (09/02/2012 @ 09:44:26) ***

    The operation failed.
    Bind result: hr = 0x8007000b. An attempt was made to load a program with an incorrect format.

    Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
    Running under executable  C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\MSTest.exe
    --- A detailed error log follows. 

    === Pre-bind state information ===
    LOG: User = David
    LOG: DisplayName = x64Production, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
     (Fully-specified)
    LOG: Appbase = file:///D:/
    LOG: Initial PrivatePath = NULL
    LOG: Dynamic Base = NULL
    LOG: Cache Base = NULL
    LOG: AppName = MSTest.exe
    Calling assembly : AnyCPUTestingx64Production, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null.
    ===
    LOG: This bind starts in default load context.
    LOG: Using application configuration file: C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\MSTest.exe.Config
    LOG: Using host configuration file: 
    LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
    LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
    LOG: Attempting download of new URL file:///D:/x64Production.DLL.
    LOG: Assembly download was successful. Attempting setup of file: D:\x64Production.dll
    LOG: Entering run-from-source setup phase.
    LOG: Assembly Name is: x64Production, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
    ERR: Failed to complete setup of assembly (hr = 0x8007000b). Probing terminated.

Has anyone else ascertained whether or not this is simply unsupported in the VS2010 mstest?