There could well be something worth investigating, but I prefer the peer reviewed literature. When I entered magnesium and asthma into PubMed's query box, I got 429 citations. I found this on the first page:
Magnesium Concentration in Acute Asthmatic Children
It's a small study, but for those who had exacerbations of asthma they found "Erythrocytic Mg levels were significantly lower during the acute asthma, and were negatively correlated with severity of exacerbation..." Erythrocytes are red blood cells. Mg is the abbreviation for magnesium.
Magnesium deficiency and asthma at PubMed gets 52 citations. Click on Review near the top of the left sidebar. There are 24 results, 23 abstracts including 3 free articles. I'd start there.
If you want to attempt a therapeutic trial of magnesium supplementation, it will more than likely be as a magnesium salt. Check its bioavailability and with your doc.
I can tell you it works. Unequivocally. It is a natural mineral and most people are deficient. If you dont want to try it and instead continue with high end severe asthma, that is your choice!