There is a group that worked out the math on using MagLev to boost a payload up to escape velocity.
The Launch Ring: A Magnetic Satellite Launch System
http://www.launchpnt.com/portfolio/aerospace/satellite-launch-ring/... refer to the 2nd photo, the launch vehicle loops around the MagLev circle as many times as necessary to bring it up to escape velocity and then it is redirected (magnetically) onto the launch track and off it goes.
This system would drastically reduce the cost of putting material into space ($750/lb versus $4,000/lb for a rocket) with a technology path toward $100/lb into orbit.
That's why I think your idea is a really good one. If you want the space-based version in the OP then you'll need a large mass object so the space craft isn't pushing the MagLev ring out of its assigned position by much. Maybe if the rings were inside of a medium-sized asteroid (with a drilled out tunnel going all the way through, sort of like an apple corer does). Then you could get massive amounts of acceleration and if the MagLev asteroid were positioned at one of the Earth-Moon Lagrange points it would be much easier to keep it in position. (can't get around the "for every action there is an equal and opposite reaction bit so it would have to be repositioned occasionally but maybe solar sails could be used to accomplish that).
I don't know how the human body would fare under the acceleration g-forces that type of system would be able to create (limited only by the power source)... Navy deep sea divers breathe a liquid mixture which protects them from the pressure. Perhaps that could work for the astronauts as they say it will allow them to take up to 20 g's of acceleration -- fighter pilots pass out around 5 g's, normal people around 3 g's.
http://en.wikipedia.org/wiki/Liquid_breathinghttp://en.wikipedia.org/wiki/Liquid_breathing#Space_travelhttp://www.thenakedscientists.com/HTML/content/latest-questions/question/2204//edit to add: Yes, you could build one on the moon. Or several. With far less gravity the escape velocity would be lower than such a system on the Earth. And since it's feasible on Earth it would definitely work on the moon. Just need enough power at the site.