Hi,
I work with kids with cerebral palsy. I have not treated anyone with opisthotonus but I remember a talk about a child with this condition. They treated this child with 24 hour positioning, predominantly in a flexed position ie. alternately sitting in his chair (he tolerated this if the tilt-in-space function was used), lying in side-lying with his neck bent slightly forward, and being held in a flexed position. Gradually the reflex diminished.
I hope that this is of some help.