Make a UNC path trusted for DLL referencing on Server2008R2

Question

I need to run MSTest on an assembly referenced by a UNC path. When I try I get a generic error, which Google tells me is trust related.The machine where MSTest runs and the network host are both Server2008R2 (in some circumstances the same machine).

For example, running this works fine:

mstest /testcontainer:c:\builds\SimpleLibraryTest.dll

But this (which is just the UNC path to the same directory -ie MSTest is running on the same machine where the dll is hosted) does not:

mstest /testcontainer:\tfs2010\builds\SimpleLibraryTest.dll

Could not load file or assembly 'file://\tfs2010\builds\SimpleLibraryTest.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)

Everything I'm finding in Google is talking about configuring trust using Mscorcfg.msc, which is deprecated in .NET 4.

How do I configure .NET 4 (on Server2008R2) to trust DLLs specified by a UNC path?

Cheers