$ otool -L /Developer/usr/bin/productsign
/Developer/usr/bin/productsign:
/System/Library/PrivateFrameworks/AppleFSCompression.framework/Versions/A/AppleFSCompression (compatibility version 1.0.0, current version 24.0.0)
/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 550.0.0)
/usr/lib/libbz2.1.0.dylib (compatibility version 1.0.0, current version 1.0.5)
/usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.3)
/System/Library/Frameworks/Foundation.framework/Versions/C/Foundation (compatibility version 300.0.0, current version 751.0.0)
/System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 36910.0.0)
/System/Library/Frameworks/CoreServices.framework/Versions/A/CoreServices (compatibility version 1.0.0, current version 44.0.0)
/usr/lib/libxar.1.dylib (compatibility version 1.0.0, current version 1.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 123.0.0)
/usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 227.0.0)
I am not sure if "AppleFSCompression.framework" is installed by Mac OS X or Xcode.
Other then that, I can not see any issues for productsign to work without Xcode 4.
As Stephane suggested, extract it from Xcode 4.2 for Snow Leo package and run it.