I've heard about a mod you can put in your guitar which basically runs your guitar through a couple of diodes, essentially working as a passive distortion. How would I go about making this? It sounds like it could possibly be really shit sounding though.

Any experience with anything like this?
Yeah, i'm not sure what the exact circuitry is but both the Black Ice sold by Stewmac and Brown Ice sold by Ormsby are basically this. All it is is some passive circuitry in a plastic housing you hook up to your tone pot in place of a capacitor and it turns it ito an overdrive.
Just looked it up and all it is is two schottky diodes in parallel with one facing the opposite direction. Pretty simple but the sound wouldn't be that great.