i ended up doing this in the Build Events, Post-build event command line(s) (guessing that maybe it would function with DOS commands, which is "clunky", but I never got a response on several forums):
copy "$(TargetDir)$(TargetName).*" "$(DevEnvDir)PrivateAssemblies\"
IF exist "c:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2010x64 exists )
IF exist "c:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2010x86 exists )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2012x64 exists )
IF exist "c:\Program Files\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2012x86 exists )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2013x64 exists )
IF exist "c:\Program Files\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 2013x86 exists )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 13.0-x64 exists )
IF exist "c:\Program Files\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 13.0-x86 exists )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files (x86)\Microsoft Visual Studio 13.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 14.0-x64 exists )
IF exist "c:\Program Files\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" ( echo Visual Studio 14.0-x86 exists )
IF exist "c:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" ( copy "$(TargetDir)$(TargetName).*" "c:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\PrivateAssemblies" )
net stop "SQL Server Reporting Services (MSSQLSERVER)"
IF exist "C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( echo SQL Server 2008-MSRS10 exists )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( copy "$(TargetDir)$(TargetName).*" "C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportServer\bin\" )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( echo SQL Server 2012-MSRS11 exists )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( copy "$(TargetDir)$(TargetName).*" "C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\bin\" )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS12.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( echo SQL Server 2014-MSRS12 exists )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS12.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( copy "$(TargetDir)$(TargetName).*" "C:\Program Files\Microsoft SQL Server\MSRS12.MSSQLSERVER\Reporting Services\ReportServer\bin\" )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( echo SQL Server 201x-MSRS13 exists )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( copy "$(TargetDir)$(TargetName).*" "C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\ReportServer\bin\" )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS14.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( echo SQL Server 201x-MSRS14 exists )
IF exist "C:\Program Files\Microsoft SQL Server\MSRS14.MSSQLSERVER\Reporting Services\ReportServer\bin\" ( copy "$(TargetDir)$(TargetName).*" "C:\Program Files\Microsoft SQL Server\MSRS14.MSSQLSERVER\Reporting Services\ReportServer\bin\" )
net start "SQL Server Reporting Services (MSSQLSERVER)"
granted that the:
copy "$(TargetDir)$(TargetName).*" "$(DevEnvDir)PrivateAssemblies\"
is in all likelihood redundant (going to happen again in the commands), but "I'm covered" (i.e. if I build the SSRS DPE in VS2013, register it in SQL Server 2012, I'm going to see the extension in VS2010 when I create a report or need the Report Query Designer to be custom, based on the (custom) DataSource selected).
also, some nice procecessor specific logic here: http://code-jedi.com/blog/post/Using-Platform-Specific-Libraries-from-a-Platform-Agnostic-Project.aspx
Rob K