I am having a hard time picturing your layout. Do you envision most of your hangs and focuses to be done from the catwalks or from ladders and lifts? Most of the black-box type spaces I have worked in have about 80% of the lighting positions not accessible by the catwalks, thus making it impractical to store gear there. Point being, if you think that most hang and focus operations will be done from ladders or lifts then you may want to consider storing your gear on the ground.
Another issue that I see with storing gear in catwalks is that often catwalks are narrow and dark. Having racks that extend out from the wall may pose a hazard in that people could walk into them. Cable racks are especially less visible when they are empty.
There is also the issue that you are storing gear over people's heads. You will have students, staff, and patrons somewhere below the catwalks most times that the
theatre is in use. You would need a pretty good
safety system to keep gear on shelves and hanging brackets. It will be a real bummer when someone walks by and inadvertently bumps one of your SeaChangers off the shelf it narrowly misses some actor's grandmother. In your scenario above you give a walking space of 24" after installing a 12" shelf. When was the last time you tried to walk in a 2' wide opening, that is a pretty tight space, barely an average shoulder width.
Also, it is a lot easier to work on gear on the floor where you have a proper work surface and tools. Keeping everything in the catwalks means that you have to schlepp it around to service, or you have bring loose tools up to the catwalks. Taking things apart with lots of parts in a catwalk is probably not the best idea. You probably want to store your gear in relative proximity to where you maintain it.
Next on the list is workspace. Catwalks are generally tight spaces in general. If you start adding shelves and storage and make them tighter, it will make it harder to work in the catwalks. Not just walking around, but you then have to think about where you can bend over, and if you are working under a shelf.
I have one catwalk where I store things, and the only reason I use it is because the catwalk is not in service. It was built in order to maintain the cold cathode system that was built into the theatre that no longer functions. Since I don't have to maintain that system, the catwalk serves no purpose except as a graveyard for old gear. It is above a solid ceiling thins the odds of anything falling into the
house are very very slim (you would need a lot of force to go through the ceiling).
The catwalk deck might also affect what I would think of storing in the catwalks. If it is made of something like solid diamond plate sheet steel i would be slightly less leery than if it were grating. Most cats have grating for a deck. In my cats, a
stage pin connector will fit through the grate, so an entire cable could fall through! We often use this to our advantage when we need to pull circuits from one position to another.
Lastly, you work in a school, you have free labor, and hauling gear around is what theatre electricians are best at.