I would be honest and tell them about it. Your honesty may pay off and they may let you for the sake of art, especially if you present it in a non-sexual context.
My thinking in this situation is that if you have to lie to accomplish something so small, it wasn't worth lying about.
Appeal to their vanity. Sweet talk them, tell them they have cute, ideal feet that would look good for this project and would really help you out. Provided this is true, and you are not lying, then its a double-gain- you were sincere and honest, and getting them to do it with less deception and more flattery is the spice which makes the accomplishment taste great. Also, you may have made someone feel good about themselves. Even if it is just their feet, a compliment is a compliment, and most people, females especially, would appriciate that (but not of it's bullshit just to get to touch them, which women are good about sensing sometimes, which is another motivator toward being honest).