An ahead stem relies on friction began the steerer and the inner diameter of the stem.
Clamping on to a threaded diameter will change the properties of the joint. It may or may not be a problem. The stem might move, it might crack (look at all those stress risers!) it might work perfectly for 20 years.
FWIW, I'd be fairly happy doing it with a nice solid stem, on a moderately heavyweight pair of forks. A 100 gram stem and a pair of lightweight or exotic forks, I'd either use an extender or the proper set up.