[Haskell-cafe] Supplying a default implementation for a typeclass based on another class

I'm in the process of writing a distributed filesystem (in haskell, yay), which of course means I'm using Binary for de/serialization. Now, that's fine enough, but for simplicity (and for wireshark), I'd like to be able to have Binary fall back on an instance based on Show/Read for any type that lacks any other Binary instance.. Well, I understand why this would be somewhere between extremely hard and impossible in haskell '98, but I'm not entirely up on all the extensions, so I thought I'd ask - given every extension implemented in ghc 6.10.1, is there any reasonable way to do this?