My partner and I are working on a small, analog device that would attach to the strings. We're thinking that for simplicity of design, attaching the pieces using magnets would be most elegant. However, I know that you could theoretically mess up pickups with magnets nearby and that strings are usually magnetic so that they interact with the magnets in the pickup. Any idea how strong a magnet would have to be to mess with the pickups? If not, we'd probably just use a hinged clip but it's less pleasant while strumming.
Put a magnet on the strings.
Listen to hideous buzzing rattle caused by the magnet not quite adhering, or if it does, the alteration in pitch caused by the additional weight.
Take magnet off again. Think up a new idea ;))
You don't need to get as far as how much this would interfere with the pickups.
The magnetic pickups in electric guitars work by inducing a small electric current when magnetic field changes. The field changes when the ferromagnetic strings move in the field, so adding an additional magnet near the pickups would at worst change the tone in similar way as changing a magnet inside pickup to different one.
There are however other problems with the approach. Just try attaching a magnet to the string and you'll see.
I think you’ll have much bigger problems with doing that. First of all it will be quite hard to get a magnet holding any load attach firmly to the string. But adding mass to the string will affect how it vibrates, it can even mess up intonation.
We do not know what your device is supposed to do, but attaching things randomly onto strings is usually not a good idea :)