Sorry, I was in a rush when I posted. I had two new techniques. I relied on the fact that if you drop water on magma where the magma is in contact with the side wall of the volcano, it will turn into obsidian and not cave-in.
For the first technique, we start off by draining the volcano. This is pretty simple, you just drop a water sandwich[1] down the middle of the volcano. It will turn all of the magma into obsidian, which caves in and falls into the SMR and disappears. Then we allow the tube to re-fill to the desired height. Finally, we drop water into the volcano, making sure that
at least one edge of the water is up against the edge of the tube. The water that is against the side of the tube will hit the magma, turn into obsidian, and will not cave-in because it is supported by the edge of the tube.
Unfortunately, water does not fall evenly, so creating a larger platform may be troublesome. If you drop all the water at once, you may end up with some holes in your platform. I did in my test. If you drop the water a little at a time, the magma may refill too quickly, not allowing you to finish the platform. I didn't test this.
For the second technique, I created a spire without draining the volcano first. I tried to duplicate this under controlled conditions, and haven't had any luck yet.
Also, to answer your earlier question "Can you make a lever which times the dwarfs exit and then very specifically does another action after a series of ticks"; I would use minecarts and pressure plates. Specifically, I would figure out how long it takes the dwarf to exit, and set up the minecart/plate accordingly.
Finally, I think I understand the spire mechanics. It's a bug. The water is pushed down with the cave-in, falling to the bottom instantly. Then the layer above the water (which pushed the water down in the first place) collapses through the water, pushing the water back up. Then, (and here is the bug), the code checks to see if it should create the obsidian, and does so, and after it creates the obsidian, it checks if the tile is blocked, and if so, it pushes the water up again.
Edit:I keep thinking I'm done, when I come up with something else. So, this method involves making the submarine taller than the distance between the SMR and the bottom of the volcano tube, and overlapping the wall of the submarine with the volcano tube. It creates a spire, but no hollow (so far). You can build the submarine over the volcano, or you can channel out the edge on the inside of the volcano to make room so that it isn't supported.
Example save at:
http://dffd.wimbli.com/file.php?id=8390(Sorry for the sad state of the fortress proper.)
I'm just picturing the goblins looking from a distance at my giant plumes of magma mist 25 z-levels above sea level and saying "Fucking Dwarves".
Edit 2:Another example save:
http://dffd.wimbli.com/file.php?id=8391Hollow spires, has an obsidian floor and 2/7 water half the time. Refills with magma the other half of the time. I have no idea what to think of this one. It's definitely the closest I've come to the dwarven submarine I promised, but I have no idea how or why it works.
For the download impaired, The top three layers are:
FFFFFFF WWWWWWW WWWWWWW
FFFFFFF WWWWWWW WFFFFFW
FFFFFFF WWFFFWW WFFFFFW
FFFFFFF WWFFFWW WFFFFFW
FFFFFFF WWFFFWW WFFFFFW
FFFFFFF WWWWWWW WFFFFFW
FFFFFFF WWWWWWW WWWWWWWand it creates a 5x5 spire with a 3x3 hollow.