[petek]

A couple of questions which pop in my head about the do-ablity of your idea:
How far away from the foot do you plan to be? (The farther away you get, the weaker any reflection of your light would be and the more interference you'd get)
Do you hope to detect the foot from an angle, or just perpendicular to it? (The more on-axis you are the better your chances will be)
Don't forget that there is a diamond plate skirt all the way around the bottom of the rack which might create a lot of reflection interference if you're looking at the lower spider feet, but it might also be a bigger target...