Does anyone know if it's possible to re-build all of ROS in 32-bit mode? Is there a system-wide way to set CMAKE_OSX_ARCHITECTURES to "i386" and rebuild?

http://www.ros.org/wiki/rosbuild#Customizing_the_build_.28debug.2C_optimizations.2C_default_build_flags.29

You should be able to create a rosconfig.cmake file in $ROS_ROOT and set any build options you want.

Josh