ATM I've got three systems on full time in this house, which augments the radiator quite nicely :-D
I like the idea about the external hd casing, but for me I'm afraid that once warmed, it would no longer be a feetsink, but a feet stink (OK, I admit it now, thats probably one of the worst puns I've ever put up).
But the principle is sound.
Turning up the heat in this house.... get up and increase the flow to the radiator, or set off instances of Seti@home ;-)