I currently have a fairly ugly and not too reliable method of sensing bolt position, it doesn't rely on bolt, but instead the thumb turn. On said thumbturn there is a reed relay style magnet, a reed is sitting on the side of the thumb-turn casing, placed optimally. It requires wires ON, or IN the door... a frame-based solution makes it 100x easier to conceal.
I was perusing the internet for tamper-detection (same system, different part) when I came accross inductive sensors. Yes, the steel bolt is conductive. But how far would it work? Would it need to be right up to the bore? If so, i'd use a optical sensor. If it would sense a residential style slauge brand bolt from 3" to determine whether the bolt is at one point, two inches maybe to it's other extent.
Sigh... I may just go with an ir prox detector. BUT, drilling to place it reduces frame strength right where it's gonna count, behind the bolt. Yes, i have a 12x spax 3 1/2" screw strike and frame reinforcements (more 12ga 3 1/2" Spax), but I wouldn't want to hurt security.