Is there a way to overwinter popcorn cassia (I'm in Maryland)?
Alternatively, I understand it can be planted from stem: is there a way to preserve a stem over winter and plant it back in Spring perhaps?
Stem cuttings are a typical way of propagating this plant. Root the cutting in a pot during the growing season.
In Maryland, these plants will need to be overwintered indoors. This is a tropical plant, so we do not have information or experience with it. Try to keep it in conditions that match those it would experience in its native region, i.e. bright sun, moist soil.
This link may prove helpful: http://wimastergardener.org/article/popcorn-cassia-senna-cassia-didymobotrya/