what would you need a magnet on the rail for? Are you trying to rig up a system to secure it to your body?
There are other options for that. I had one on my mp7 for the short time I had it. Basically it mounts on the rail with a male end, and there's a female recepticle that was mounted with molle on my gear some where, and I could just clip it in out of the way.
The only issue was that it would pivot. You would need a magnet strong enough to stabilize the weight of your aeg so A) it won't fall off, B) be loose that it would get in the way while you're doing the task you want to do.
A proper 2 pt sling should be fine if you sling the weapon across your back and cinch it down. I've climbed ladders and done all sorts of things with a full length g36 slung on my back. I use a convertible 2-1 pt so when I'm walking long distances, I change it to 2 and sling it on my back, when I'm moving slowly I have it in 1pt so I can drop it and draw a pistol if I need to.
If you are attaching stuff TO your gun via magnet, then have at it, just make sure they're strong enough to hold whatever you want to stick on the gun. Just remember some heavier BBs have metal filler in them to increase their weight, so they can be affected by strong magnetic fields.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.