I know of no medical studies that have shown Yin Yoga to be helpful with asthma but there is anecdotal evidence that yoga in general can assist with this condition. There are lots of articles on this topic and some of them were written by people with the condition. Check out this really good article in
Yoga Journal, for example.
From an Eastern point of view, asthma has been treated through acupuncture treatments, although there is conflicting evidence from clinical studies as to whether this really works or not. One
web site does offer several resources that you may want to look into deeper, but it is rather technical and I would suggest that if you really want to investigate a TCM solution to seek out a professional acupuncturist or TCM doctor. The common TCM treatments do seem to focus on the upper back acupuncture points. You may try experimenting with using acupressure via your Yin Yoga practice to stimulate these points: Anahatasana, Frog, Sphinx, Seal, Saddle and Swan as well as the flexion poses of Butterfly and Caterpillar.
More substantive (ie measurable) benefit has been claimed through regulating and retraining your breath (see the Yoga Journal article again and then read the
Yin-side of Breathing posted in my
Newsletter #5.) The work of Dr. Gay Hendricks, author of
Conscious Breathing might be the best overall resource you can find and he does mention yoga several times. What you can do is adopt his breath-work exercises while you are in your Yin Yoga postures. Why not do the above upper back Yin Yoga poses while working with the breath: lengthening and easing the effort you breathe with.
I hope some of this helps. Let us know how it goes.
Cheers
Bernie