I guess it would be safe-ish. Just don't rock about too much because the steel cables could pull out of the ceiling, or if you swung the bed out of the field it could flip or fall down.
Hmm ...if it all has the same charge on both sides then if you flip it it shouldn't come down and crush you. It'd give a whole new meaning to 'rocking the bed' eh? I'm wondering what if they gave it a bit of an upwards curve on the ends and curved the magnet too ...maybe that'd hold it in place a bit.
I suppose that depends on whether the cables are attached to the ceiling or the floor. I assume the floor, as to keep the bed from moving erratically. If that's the case, the weight of the bodies would reduce the tension on the cables, not increase it.
If a magnet is pushing up from the ground and the rope isn't at full tension, it's going to shoot the bed to the side. That is unless the force exerted on each part of the bed is IDENTICAL. Don't think that's going to happen in this case.