I've confirmed that it still works on 10.5. I was less successful than you on 10.4:
[dalejos-powerbook-g4-15:/Volumes/Data/tests] dalejo% /llvm-gcc4.2-2.2-ppc-darwin8.11.0/bin/llvm-g++ ctor1.C
/usr/bin/ld: unknown flag: -macosx_version_min
collect2: ld returned 1 exit status
This is a ppc32 machine running 10.4.11, but does not have a recent tools release installed on it. What's the prerequisite here?
I think the problem with EH is that you're getting the unwinding library that was built as part of the llvm-gcc build. That one will not work. We want to pick up that library from a gcc build (or it may work just to remove the one that's there, if we require some other tools be installed). It seems to use libgcc_s.10.[45].dylib on 10.5 and libgcc_s.dylib on 10.4. But it may be that 4.0 vs 4.2 is another variable, I'm not in a position to test all combinations.
I've used llvm-gcc to compile as far as a .o, then used g++ to link, and that works. So I think this is the problem.