Do you have the plywood directly behind the flannel covering? This is just a suggestion: take the flannel off and glue onto the plywood a same size sheet of one inch blue insulation. Then put the flannel back on the insulation. That way you can shove pins directly into the wall. You can also stick things that are not too heavy onto the flannel. You say there is a layer of batting under the flannel, but I'm not sure that is thick enough to pin to.
You adaptation of the sliding rail is genius.
Marcia