Extrusion in Baluster Panel family, or nested generic
model in baluster panel family. It might
take some adjustment between the project and family, but this is definitely a
possibility. If the railing geometry is
curved or breaks to many landings this method may make sense more than #2.
this as In-Place Component in Project
If the stair is relatively finalized, you may consider
creating an in-place component in the project.
You should be able to set the active work plane to a face on the
railing, and then model this condition as needed. This may be the easiest method to create this
type of geometry.
I hope some of this may be helpful; if you have any follow-up
questions let us know.