STATUS UPDATE: July 17
As you can perhaps see from the image, today my project was mostly to test the fill behavior of the reservoirs. As it turns out, I learned some interesting stuff - maybe it was generally known, but I certainly hadn't had reason to look it up prior to this point.
-A screw pump, when pumping into a confined space, will pump until the space is as full as it can get and then cease to move liquid forward. The other possibility I had considered was that it would continue to pump forward, either pressurizing or destroying the liquid that passed through. This is a much more advantageous option for me, as it allows the system as a whole to "fill" and maintain maximal magma storage.
-Magma does not (whew) leak through open space above aforementioned confined spaces when there is a power conduit of some sort (in this case, windmills above gears/pumps) filling the space. I cannot testify to what would happen without the power linkage, or with water, or in any other direction, and am not responsible for the ramifications if someone decides to overpressure a small room in their own fort to see what happens.
I also decided on a lever grid setup. I have three lever rooms, spaced where I found a large enough uncluttered area throughout the fort. They are all laid out in a miniature diagram representing the hexagonal layout of the reservoirs themselves. The first one is the Fire Control Room. This one has a lever for every reservoir, and opens the bridge at the bottom to dispense death and destruction.
The second two rooms concern the flow of magma throughout the grid. After lots of thought and whiskey, I decided to arrange the grid in a more or less concentric pattern for as long as I could. Every hex is designated by its distance from my fortress entrance. The magma pipes that connect WITHIN a distance ring are the primary outflow of the reservoir, and are controlled from the Upper Flow Control Room. The magma pipes that feed BETWEEN distance rings are the secondary outflows, and are controlled from the Lower Flow Control Room. Not every reservoir has a secondary outflow, so the grid is a bit patchy. I'm thinking of subbing in orthoclase floor tiles to represent the leverless stations just for visual completeness.
Lastly, I dedicated some time to roofing over and otherwise modifying my fortress entrance so that I could literally dump lava right on top of it and be good to go, as well as ensuring that the entryway was raised up so as to deny entry to any incoming magma flows without the need for gate systems that I will inevitably fail to activate.