Restricting airflow isn't the same as restricting oxygen uptake, unfortunately. I see people running around my neighborhood with those silly masks, huffing and puffing, and I feel very sorry for them, as they've been duped.
Moreover, the research is all over the place on whether or not simulated altitude at normal pressures (such as that provided by a tent and a generator, which just strips out oxygen and replaces it with more nitrogen) helps with work capacity at real altitude. In my experience (I've got a generator/tent, and I'm sleeping in it now at about PP "altitude,") it doesn't help much with work capacity. It does, however, appear to help with acclimatization as it regards altitude sickness symptoms. Research seems to confirm this as well. I recently traveled to 12,000 ft and spent hours there without a bit of headache or other issue, and I live at sea level, but use the tent.
Whether or not this benefit is worth the cost is a good question. Any number of race day issues can more than overcome any advantage this might offer.