Link to forum thread:
http://forum.unity3d.com/threads/build-error-with-wsa-plugin.300913/

I'm writing a WSA plugin with a reference to UnityPlayer, so I could use UnityPlayer.AppCallbacks to run code in UIThread or AppThread from within the plugin.

reference
\Unity\Editor\Data\PlaybackEngines\metrosupport\Players\WindowsPhone81\ARM\master\UnityPlayer.winmd

This build error log in Unity 4.6.3f1

Error building Player: Exception: Failed to run Reference Rewriter with cmdline --target="Temp/StagingArea\DeviceUtils.dll" --framework="C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\WindowsPhoneApp\v8.1" --platform="C:\Program Files (x86)\Windows Phone Kits\8.1\References\CommonConfiguration\Neutral\Windows.winmd" --support="Temp/StagingArea\WinRTLegacy.dll" --supportpartialns=Unity.Partial --system=System --dbg=pdb --alt=System.Xml.Serialization --ignore=System.IConvertible,mscorlib.[Temp/StagingArea\DeviceUtils.dll]
Catastrophic failure while running rrw: Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'UnityPlayer, Version=255.255.255.255, Culture=neutral, PublicKeyToken=null'
at Unity.ReferenceRewriter.RewriteContext.RewriteResolver.ResolveAndRegisterWinmd(AssemblyNameReference name)
at Unity.ReferenceRewriter.RewriteContext.RewriteResolver.Resolve(AssemblyNameReference name)
at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
at Unity.ReferenceRewriter.RewriteTypeReferences.Visit(TypeReference type, String referencingEntityName)
at Unity.ReferenceRewriter.ReferenceDispatcher.DispatchFields(TypeDefinition type, String referencingEntityName)
at Unity.ReferenceRewriter.ReferenceDispatcher.Dispatch(TypeDefinition type)
at Unity.ReferenceRewriter.ReferenceDispatcher.Dispatch()
at Unity.ReferenceRewriter.RewriteOperation.Execute(RewriteContext context)
at Unity.ReferenceRewriter.Program.Main(String[] args)

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Won't fix reason:
Users shouldn't reference unityplayer.winmd;