I don't know how to properly do it. I can give some instructions:
A) install gcc-4.7 toolchain (http://wiki.maemo.org/CSSU-thumb_too....7.2-linaro%29 )
B) enable my wheezy repo (maybe squeeze also, until I got all of them also for wheezy)
C) if old binaries from sb is preventing to use new arm-binaries, hack them outside of scratchbox. e.g sudo mv /scratchbox/tools/bin/scons /scratchbox/tools/bin/scons_orig
sudo mv /scratchbox/devkits/git/bin/git /scratchbox/devkits/git/bin/git_orig