First post.. been lurking a while. Anyway, I reversed the checksum for rage gamesaves and made a little app. It works for all files inside the save con apart from instance*.rage files which require decompression first.
Quick look at the source is here: http://pastebin.com/ZTpnjKpJ
Binaries and full source here: http://dl.dropbox.com/u/37569004/ragecrc.zip
It's a console app but usage is simple: ragecrc.exe <save file>