Warn-as-error for MSB3277: Found conflicts between different versions of the same dependent assembly that could not be resolved

No happy answer to give you here. The TreatWarningsAsErrors property only affects the C# and VB.NET compiler (not C++), it determines the value of their /warnaserror command line option.

But MSB3277 is generated by an MSBuild task, ResolveAssemblyReference, its internal LogResult() method generates the diagnostic. The only property the class has that helps treat warnings as errors conditionally is WarnOrErrorOnTargetArchitectureMismatch, not what you are trying to achieve. You can have a look-see for yourself with a decompiler, look at C:\Program Files (x86)\MSBuild\12.0\Bin\Microsoft.Build.Tasks.v12.0.dll. The resource name for the localized MSB3277 message is "ResolveAssemblyReference.FoundConflicts".

So only way to get ahead here is to write a little utility that parses the log file and looks for the warning.