I think that NASA's idea is interesting...very challeging, as other have noted, but worth it if it could be done.
I have been toying with the idea for an SF story using comets. Spaceships would wait for a comet to come by, then embed themselves into the tail of the comet, and use some kind of ramjet propulsion to accelerate out of the inner solar system. Obviously comet tails are not dense at all (a less dense vacuum than what can be made on earth) but the ion tail should be manipulatable.
Anyway, in the story, people in their spaceships end up flying out in more-or-less random directions, and hoping to find something interesting in the process.