As near as I can determine, while building a project in Xcode, the Rez tool will ignore the SDKROOT_i386 & SDKROOT_ppc build settings and will instead always look in /System/Library/Frameworks for things like Carbon.r, etc.
Is this correct?
For those who are interested, there is a workaround for what I would consider to be a bug -
Set 'Other Rez Flags' to: $(REZ_FRAMEWORK_PATH_$(CURRENT_ARCH))
Then, define two custom build settings:
REZ_FRAMEWORK_PATH_i386 = -F /path/to/framework
REZ_FRAMEWORK_PATH_ppc = -F /path/to/framework
and set them to the framework path you wish to use with the -F flag.
If you want to see where Rez is finding things like Carbon.r, add the '-p' flag to the 'Other Rez Flags' build setting. Doing so will generate a lot of errors (why, I do not know...another bug?), but you will be able to see in the build results exactly what Rez is doing.