It worked fine for me, but then I didn't try to do anything fancy. The data-dir and prefix you mention is where cabal installs all my packages, FWIW. Not being able to access them would be a problem! You can customize it with --data-dir and --prefix on the cabal install command line, I think. Not sure if that's the reason your initial installation wasn't working though...

Also, if a library fails to build because of your ghc version, you can install a specific version of that library instead. E.g. for haddock you see that the failing version was 2.4.2 but if you leave out the version number cabal will fetch the latest one for you, which isn't compatible with GHC 6.10.