Why don't you want to use a quill adapter?

I would worry about clamping to threads. Instead of the stem tightening on a nice smooth piece of metal with lots of contact area, you're limiting your contact to the ridges of the threads. While the threads would probably smash down with enough clamping force, you'd then have a smaller diameter at the stem than the original fork. Might make keeping the stem tight problematic.

Just speculation.