My workaround was to rebuild the cabinet file myself using the existing cabinet file and the working RSWebParts.cab file from a SSRS 2008 install.
- Extract the localization resources from the working RSWebParts.cab for 2008 R2 or earlier into a directory.
- Extract RSWebParts.cab from 2012 SP1 into the same directory and recreate folder structure per manifest.xml file.
- In the same directory as the files, create a Diamond Directive File (rswebparts.ddf) with the following contents:
.OPTION EXPLICIT ; Will Generate errors for mistakes
;.Set CompressionType=MSZIP ;** All files are compressed to cabinet files
.Set DiskDirectoryTemplate=CDROM;
.Set DiskDirectory1=;
.Set Cabinet=on
.Set CabinetNameTemplate=RSWebParts.cab
manifest.xml
report_explorer.gif
report_viewer.gif
RSWebParts.dll
RSWebParts.inf
RSWebParts.resources.dll
SPExplorer.dwp
SPViewer.dwp
.Set DestinationDir=de
de\RSWebParts.resources.dll
.Set DestinationDir=es
es\RSWebParts.resources.dll
.Set DestinationDir=fr
fr\RSWebParts.resources.dll
.Set DestinationDir=it
it\RSWebParts.resources.dll
.Set DestinationDir=ja
ja\RSWebParts.resources.dll
.Set DestinationDir=ko
ko\RSWebParts.resources.dll
.Set DestinationDir=pt
pt\RSWebParts.resources.dll
.Set DestinationDir=ru
ru\RSWebParts.resources.dll
.Set DestinationDir=zh-chs
zh-chs\RSWebParts.resources.dll
.Set DestinationDir=zh-cht
zh-cht\RSWebParts.resources.dll
Finally, open a Command Prompt and run makecab /f (your ddf location)
This will create an RSWebParts.cab file in the directory that is properly structured. Copy this to your Sharepoint server and install it per the instructions on MSDN.