The setup would probably work ok and serve it's purpose as a temporary low cost solution until a new fork arrives. Seems like the main question is if it might mess up the nice threadless headset. Maybe some locktite or something in the threads of the steerer where it may sit would help.
Different from this situation, but I did a pretty desperate modification to allow riding a too short threaded fork with threaded setup. I cut off the top of another threaded steerer and slipped it inverted onto the quill stem and used the threads at the bottom for the upper race, then shoved the stem with it down into the fork and tightened the quill. The overall length of the cut off steerer section and the length of the threaded part of it were dependent on where the stem ended up as far as it's height and acceptable insertion into the fork steerer (this section buts up against the crook of the stem and can't hit the top of the fork steerer). Kind of hard to explain and visualize, but it "works".
This was only meant as a very temporary fix to allow testing the ride to determine if it was worth keeping and looking for another fork, and it is, I really like the ride with this fork as a reference for a replacement. It's a secondary bike and I don't consider it very safe (if I loosen the stem bolt the fork falls out) and ride it accordingly while keeping my eye out for an acceptable longer fork.
Anyway, whatever you do try not to mess up the new headset.