> You could presumably look at using a small asteroid to provide the bulk of the "cycler", offering plenty of shielding for the trip, you'd only have to get it into position once - far less fuel requirement.

Or maybe even something like, as some have suggested (not me!), Phobos:

http://enterprisemission.com/Phobos.html

Warning: it's kind of out there. At the very least, it's perhaps interesting as speculative sci-fi.