Will, I've used two methods. 1 - The eyebolt in a vise (threads protected with leather from the jaws on each side,  threads extending 1/2" above the jaws, and a small fine tooth hacksaw to make the slot. the slot can be made wider with a narrow file if needed. Jam nut may not be able to be left on the bolt to clean up the threads, depending on your jaw depth (It can prevent the jaws from closing on the threaded section of the bolt, if so, a 1/4-20 die is used to clean threads if necessary). 2 - At my Northern shop, I have a mini table saw with a 4" carbide blade that I use to cut the slot with the jam nut on the bolt. The blade is like an abrasive disk, so I can widen the slot if needed.

Will, forgot to say, I always use stainless steel eyebolts for strength and that "chromed" look.

Will, I use an electric hand drill for the 1/4" hole through the headstock, here is a front and back view on a bedpan 1 string eyebolt tuner. First the front...

and back