Would curcumin be a viable herbal remedies for asthma?

Adjunct only. There's no reason to think it will work. Asthma is a poorly-understood disease involving ongoing inflammation and kills a few thousand folks yearly. Tobacco (including passive smoking) and neglect of scientific treatment cause these deaths. Severe disease calls for glucocorticoids. Today's stuff actually works; you're welcome to experiment with diet changes and little herbal remedies.
Lifestyle change. It does help reduce inflammation, although the dosage is not known. Getting 30 -45 min of sunshine at noon daily or taking vitamin D3 10, 000 IU/day and taking a good probiotic (not yogurt based) will help too. Avoiding all milk products is very important since the milk protein causes respiratory tract inflammation. Yoga breathing exercises 10-20 min/day help too.