In case anyone's interested, I just wrote a blog post on how to add a custom Clang build as a compiler in Xcode. http://belkadan.com/blog/2011/07/Using-Clang-from-SVN-in-Xcode/
Note that this is not the same as using a custom *analyzer*. For that, you still want to use the set-xcode-analyzer script in tools/scan-build. The two procedures are orthogonal.