My scaffolding will likely be obsidian, constructed, and then removed to leave only the cast obsidian. My casting style for quick scaffold removal involves a lot of leftover obsidian boulders. Basically, my casting will look like this, for a walkway. All side-views:
Before:
# #
# #
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Casting:
#+++++#
#+++++#
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Sculpting:
# +++ #
# #
#######
Collapse:
+++
This is so that I can collapse the whole scaffolding quickly instead of deconstructing the bits of it piece at a time. Needless to say this will generate an enormous amount of leftover boulders, for which I intend to train masons to make fantastic tables and other furniture to adorn the fortress, and to build scaffolding so that I won't have to mine up new boulders to do the job. Also, needless to say, the ground level is going to be absolutely pounded to death with constant cave-ins after casting is done. The only thing I'll have to be very careful with, is ensuring I don't cave-in a section onto an existing walkway, as this could result in a whole lot of Fun. (A few dwarves will be sequestered underground to man the primary magma pumps and to ensure against total cave-in catastrophe.)
For feeding and boozing, I'll simply have farms. Constructed tiles that have been muddied will support a farm - I've tested this to be sure. One in particular of the hanging blocks is going to house a few farms, while another will have a few burrow-restricted farmer's workshops, and another will have a still and kitchen. It should all be neat and orderly, I do hope.
For double-fluid ducts, do you mean one side for magma and the other for water? That would be possible, though require larger pathways. However, due to several complications, I may do that anyways. The single biggest restriction on complexity is that windmills must have their center tile clear to the sky. This means I can't go very high without getting creative, otherwise the windmill sections will overlap and the lower sections won't work. If I double-up the size of each pump block, I can halve the number of total blocks, and that's a change in my favor. Plus it sounds very cool to have the fluids running alongside one another like that.
Now some questions for additions: What would be fun to do with all this height? Since I'm going to have running fluids, I can quite easily manage obsidian generators in the sky and produce cave-in cannons. Even massive cannons that drop whole housing blocks onto the heads of enemies. not to mention I'll have a lot of room to let fluids freefall to the surface. I plan to turn the surface level into a gnarled wasteland of dropped boulders and misshapen drainage pipes. It should also be hilarious for an adventurer to move through!