The thing with a disk brake is that the torque from braking is transfered to a single blade, at the point where the tube section is the weakest (the taper). Hence the unraking. To match the resistance from caliper brakes you would need a disk-side blade with twice the normal wall and with no taper. That may be a bit excessive (i.e. normal blades may be overbuilt), but I wouldn't risk it: you don't want the fork to fail during an emergency braking (for casual riding it's probably okay). So I don't think that tandem stay is up to the task. A (unicrown) blade tapering to a 22x1.0 section is probably the way to go for a front disk. Or you need a long reaction arm that would likely be as heavy?