@Adrian May if you want that much backward compatibility you probably need to move to an operating system that suports it.
NixOS, already mentioned by another commentator, would be my recommendation if you really need the backward compatibility I just finished compiling both Flippi and WashNGo with ghc-7.0.4.
I do not want the GHC maintainers to be tied down by maintenance concerns any more then they already are, there time is more productively spent on concerns other then increasing backward compatibility.
For anyone who works on GHC thank you for helping support such a great ecosystem that enables this level of experimentation and rate of new features.
Patrick Wheeler