The way to do this is described on a wiki page in the developers corner: http://hackage.haskell.org/trac/ghc/wiki/Debugging/InstallingPackagesInplace (scroll down to "Plan B" on this page) Thanks, that helped.
What I should have mentioned is that I am sandboxing my GHC installations, so whenever I compile HEAD I can switch to a different sandbox and install the new compiler within that sandbox. Now I can just install packages into user database without worrying that my original database gets cluttered with duplicate libraries. I managed to install some of the packages using plan B. This is tedious (I'd rather have dependencies resolved, downloaded and installed automatically by cabal) but works as long as packages from hackage are compatible with HEAD. Unfortunately some of them are not, which prevents me from installing libraries that I need (like blaze-builder) but I guess that can't be helped. Janek