This can absolutely be done. I'm actually hoping to be starting this in a month or so on my first build. I just need to locate some good cow bone.
Once you've found this cowbone you'll need to clean out it's marrow. You can do this either via the dog method, or with a straightened coat hanger. Once this bit is done you need to immerse it in some water. You have three options here you can mix in some ammonia, dishwashing detergent or leave the water au naturel. The ammonia method works best, and fastest but in order to do it you'll need a good exhaust hood above your stove.
Once you've decided which method to use bring the liquid to a boil, and then reduce the heat and simmer. 30-40 minutes if you're using the ammonia/water mixture, 50-90 minutes soap/water mixture or up to two hours if you're using plain old water -- the benefit to this method is you'll end up with a nice soup stock once you're done. The point of the boiling process is to cook away the soft tissue and begin the degreasing process.
Once the amount of time neccessary for your chosen method passes, you'll need to cool the bone to room temperature and then under a stream of running water remove the remaining soft tissue. You can use finger nails, a stiff brush, a child you don't particularly like etc. You can always throw the bone back into the pot and simmer it for a bit if this part is causing a few problems. Once the bone is nice and clean let it dry for a couple days, and then using a band saw cut it up into pieces slightly larger than the nut you intend to use. Then allow this to try for another 48-72 hours longer if you live in a particularly humid area.
Once this is done you must finish degreasing the bone. This step is absolutely crucial and must be done 100% correctly or over time grease will leech from the nut into the wood ruining your guitar.
To properly degrese the bone immerse your oversized blanks into ten volumes of white gas (naptha) for 1-3 weeks. You have to keep in mind that white gas is really flammable (and fun when you're at camp) so the degreasign process should take place outside, in an enclosed glass container in the shade and away from buildings. A greasy bone will discolour the gas in a day or two so you'll have to be changing the gas every few days during the 1-3 weeks. For project guitarists, who are only doing this as a hobby making 1-2 guitars a year, the safest (and easiest) way to get rid of the gas is to let it dissapate into the atmosphere. If you plan on going into full scale production you should contact local athorities to find out the best way to dispose of large amounts of gas.
If after 3 weeks you're still seeing translucent spots on the bone, throw it back into some more gas for a few days till they are all gone. But once you're satisfied the blanks are all good to go, rinse them in some clean white gas and air dry. These bones will dry quite quickly, in under an hour. If it doesn't, there is still some grease in there so back into the bath with it.
You now have a natural bone nut to play with. Enjoy.