I found pairs of folding shelf brackets on Amazon for $9 a pair plus shipping. Three pairs are on order, or in other words three folding backers for each 10 foot section of shelf. Note that once these folding brackets were installed I placed an order for 4 additional brackets allowing me to support each 10 foot section of the layout with 5 brackets.
The following image shows the wall on which the shelves will be mounted. Note that the wall bumps out 3 inches about half way down the wall. That’s where the depth of the benchwork shrinks from 12″ to 9″.
As can be seen from the photo, this is a nearly finished wall that is only missing a bit of trim, The benchwork will be mounted near the top of the wainscoting. The following image is a close up of the trim at the top of the wainscoting.
To secure a bracket, a vertical 1″x4″ will be screwed into the wainscoting butting up against the lower portion of the trim. The brackets will be screwed into this 1″ x 4″ cleat using 1 1/2″ screws. Brackets will be mounted 3/4″ below the top trim piece so when the shelf layout is rotated into horizontal position, the Homasote will be level with the top trim piece. To facilitate this, the back shelf edge piece will be mounted horizontally rather than vertically. That means the sandwich where the shelf mounts to the wall will be 1 1/2″ thick, 3/4″ of pine or fir and 3/4″ of Homasote. The top portion of the shelf bracket will be screwed into a cross member extending from the rear to the front of the shelf as shown in the earlier drawings.
Because the shelf will be fairly light (mostly Homasote plus a little pine or fir) three brackets should be sufficient to support a 10′ shelf. If the shelf is not totally solid, diagonal elements can be run from the cleats to the front edge of the shelf to provide further stability and support.