Thanks, everyone, for the feedback.
you mentioned celebrating magma piston teleportation, but did you get all your magma for the cast obsidian just from that?
No, there was actually surface lava available, so I just pumped that up to make the block. I'd have to do some pumping if I used a teleporting piston to get it up, anyway, since the block rises above the highest natural point in the region.
I salute you. If I was a dwarf, I might bear your children. It may be tough, being a male, but I will find some !!SCIENCE!!ey way to do it. Your advances in !!SCIENCE!! will be passed down generations.
The !!SCIENCE!! award here goes to Fenwah for
Method for transporting magma up z-levels without pumps. Go to your local university !!SCIENCE!! library and read about it in the Journal of Dwarven !!PHYSICS!! today! I can take credit only for artistic engineering.
Very, very awesome. Kudos to you. Sooo.... please explain how?
The !!SCIENCE!! is explained at the above link. The lava plume was shaped into letters like this: Under the block, there are 15 floors that were hollowed out, except for the lettering. This is the actual piston. The obsidian block at the top isn't really functional, except as filler to make sure that the lettering was obscured before the cave-in, and that the lava plume would rise well above ground level.

Below that was a 15 level magma reservoir that also fit the lettering exactly*:

(*I did make a mistake and accidentally dug out
one extra tile on one level. Being in the middle of all this, it really wasn't fixable. I could have just left it and ended up with a bit of magma trapped there after the event, but decided instead to plug it with a jewel encrusted platinum floodgate. I'm just glad I didn't dig anything I shouldn't on any of the piston levels. That would have been a disaster.)
In between was the support layer, with the block and letter piston held up by the single support in the middle:

You can also see, on the right side of the upper edge, the end of the passage I cut to tap into my magma source. Rocks are left showing the positions of the walls used to direct this magma to the various floor grates to fill the reservoirs below. This alone took years of game time, even after setting up a pump to pressurize the flow. (It could still only pump as fast as the pump's source was replenished. It really should have been closer to the actual lava tube.)
Since it was necessary to remove all stairs from inside the magma reservoir (they would block the cave-in), I needed a way out for my miners, at the bottom. And that meant I had to then fill that escape back in with constructed walls. I decided to hollow out the whole area and fill it all back in with constructed walls, just to make it nice and tidy:

The lettering is 365 units of steel, and the background is marble blocks. Just 'cause.
Bravo! Mind if I use that "Merry Dwarfmas" pic as background on my desktop?
Not at all.
For the love of all things dwarfy please tell me you have copies of that save so you can watch that over and over and over again.
No, I deleted it immediately and didn't tell anyone it ever happened. Only the cat and I will ever know about it.

That is very impressive, do you have a video of it?
The video I took leading up to the cave-in came out glitchy, so I didn't do one for after the cave-in. I could turn off TTF to see if it works any better and run it from the moment after the cave-in. I
could even turn off pause on cave-ins and go back to show the whole thing in one continuous shot, but I'm a bit loath to do so because it took a couple of hours to do the cave-in calculations.