"I don't think that the required advances would be minor in any way. "

Certainly minor compared to a "fuel-less" drive or FTL.

The basic theory is to take a metallic asteroid that is about cylindrical, bore a hole in the long axis, pack it with ice ( from Saturn's rings probably) and seal both ends. Make a large mirror ( use the material you will later make a light sail of) and heat the entire asteroid until the metal is malleable and the water turns to steam, puffing the asteroid out into the shell of your ship. add the motor and life support, attach the light sail to get you up to ram speeds, and go where you want.

http://en.wikipedia.org/wiki/Bussard_ramjet