I really like the idea of using the tube extender. If you had a steel steerer and a steel extender, you could actually weld it and then grind it. Lower weight.

As for having a free-floating spacer under the stem where one of the tightening bolts is, it probably is a bit better than tightening a clamp on air (the machinist in me has a big negative reaction to tightening anything onto air). But it's suboptimal and I think dangerous.

Bettter to use the extender idea - this has the stem clamps tightening on an integrated tube.