There are not many studies on nebulizing magnesium and reduction of pulmonary hypertension. However a Cochrane review done in 2007 investigated the studies available showing magnesium sulfate's benefits on pulmonary hypertension. They determined that the known effects of magnesium on blood vessel relaxation might indeed prove beneficial in this condition, but that more studies are needed and should be done before recommendations could be made. They basically looked only at intervenous injection of magnesium and not nebulization.
A few other studies have shown that while magnesium can lower the tension in pulmonary arteries, it will also lower this same resistance throughout the body and they speculate that this could be either helpful or harmful.
However many of the properties of magnesium chloride make it seem very likely to be of help in this situation.