There's a lot more than your conscious mind involved when it comes to being aware of parts of your body. There is something called the autonomic nervous system, and there is also something called the sympathetic nervous system. These might be the same, or related. There are also sensors called proprioreceptors all over your body that report the position and condition of that part of your body, not to your conscious mind but to lower level centres like your brain stem. I think there are even very low level systems that just go from your leg to your spine and don't even make it all the way to the brain. There are nerve ganglia in your spine that deal with certain signals that need fast response like for balance, and quick jerking motions in emergencies. There is also the lymph system and the circulatory system. These all have their own sensor networks and most of it is NOT under conscious control. Hence, even if you tried with your mind to "fool" your body into thinking you had no leg, all the other systems would not be fooled, and they would eventually overwhelm your thoughts and you would have to "give up" and agree with the rest of your body's systems that you do indeed have a leg. This would happen long before it fell off. Of course if you *physically* did something to isolate your leg, such as tying it off with a rope so that blood could not flow, eventually it would die and probably get infected. Likely the infection would spread to your whole body and kill you before the leg drops off, however.
I'd also add that there are numerous sensory and motor "maps" of our body parts in the cerebral cortex too. They would still be receiving information of the sort you described from peripheral receptors, and they would also be receiving sensory information every time the leg was touched, got cold, moved, itched, etc., whether you wanted them to or not.
In the case where someone really does lose a leg or arm due to an accident or disease, the person nearly always feels that it still there (the so-called "phantom limb"). That's because the map of the body in the brain is still whole, even if the body part is no longer attached. Sometimes these phantoms shrink and go away, but sometimes they stay for years and are excruciatingly painful. No amount of "willing" seems to make the phantom go away, so, by extension, I doubt you could succeed with your "reverse willing" exercise either.
There is another phenomenon too which afflicts some people who have lesions in the parietal cortex of the brain (where many of those body "maps" are found). They can have a perfectly intact leg, for instance, but it will no longer feel like it is part of their bodies, even if they look right at it. They sometimes become extremely frustrated and disgusted because somebody else's leg keeps following them around. Intellectually, they know it's nonsense to feel that way, but they can't stop feeling so anyway. There's a good description of this in Oliver Sacks' book, The Man Who Mistook His Wife for a Hat.
making a small donation to science.ca.