I'm not able to build a Windows Store application targeting 8.1 and using PostSharp. Here's info from the build output:
1>PostSharp30:
1> Creating directory "obj\Debug\Before-PostSharp".
1> Creating directory "obj\Debug\PostSharp".
1> Starting the pipe server: "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe /tp "postsharp-S-1-5-21-2410378251-3881919896-3453527754-1001-4.0-x86-release-3.0.33-ced32c92e0fcafaa" /config "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe.config"".
1> : info : Executing PostSharp 3.0 [3.0.33.0, 32 bit, CLR 4.5, Release]
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): warning : Connection unexpectly closed by the server.
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): warning : Retrying to execute the pipe server.
1> Starting the pipe server: "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe /tp "postsharp-S-1-5-21-2410378251-3881919896-3453527754-1001-4.0-x86-release-3.0.33-ced32c92e0fcafaa" /config "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe.config"".
1> : info : Executing PostSharp 3.0 [3.0.33.0, 32 bit, CLR 4.5, Release]
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): warning : Connection unexpectly closed by the server.
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): warning : Retrying to execute the pipe server.
1> Starting the pipe server: "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe /tp "postsharp-S-1-5-21-2410378251-3881919896-3453527754-1001-4.0-x86-release-3.0.33-ced32c92e0fcafaa" /config "C:\ProgramData\PostSharp\3.0.33\bin.Release\postsharp.srv.4.0-x86.exe.config"".
1> : info : Executing PostSharp 3.0 [3.0.33.0, 32 bit, CLR 4.5, Release]
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): warning : Connection unexpectly closed by the server.
1>c:\users\msdn\documents\visual studio 2013\Projects\App12\packages\PostSharp.3.0.33\tools\PostSharp.targets(298,5): error : Error connecting to the pipe server. See previous warnings for details.
The error can be reproduced in a new WinStore app in VS2013 by installing the PostSharp 3.0.33 NuGet package and then trying to build.
Windows Store 8.1 is not yet supported in this build.