Ask the framebuilder if using a thicker walled steerer tube would work. A standard steerer tube appears to have a maximum wall thickness of .090" and is made of crmo. You can get 1 inch crmo tubing up to .188 inch wall thickness. You would need to use a threadless stem and probably a shim to 1 1/8" over the steerer tube to make the stem fit and have a nut welded in to the top of the steerer to allow the stem cap to be tightened to set the preload as a star nut would not fit, just a thought.

With the cost of the rear disc dropout conversion and the custom fork it would probably end up costing more than a new bike.