And it works too!
Just need to machine a spacer for the break plate so it fits onto the axle I plan on using. Sadly it will not work with my rigid frame, the upper brace contacts the bracket and there is not enough clearance for both. I could probably make it work with a little grinding to my frame but I like my rigid frame more than I like the idea of having a rear disk break.
Build info - I chose a bicycle disk and caliper for the rear wheel, I ended up running a cable type break vs a hydraulic system just because it will be easier and I have a larger option when it comes to lever choices. I started the process by removing the ribs on the pedal side of the hub on the lathe. I also faced the hub so the side will be perfectly aligned with the wheel. the original bolt spacing for the disk was too small to clear the hub, I put it on the lathe and removed the offending parts. Then I brought the disk over to the vertical mill and set it up on the indexing head (a device that allows very precise 1/2° rotations of the object clamped into it) to drill 6 new mounting holes 60° apart in the disk. I then drilled 6 identically positioned holes into the hub, then tapped said holes. The disk is bolted to the hub with spacers to allow the caliper to clear the wheel.