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[Alastar]

I recently stumbled across something so basic I was surprised I had neither seen it nor thought of it before.
Or maybe it's so simple that nobody found it worth talking about.

X>>>>>X
X>>>>>X
X>>>>>X

Essentially, just a perforated floor accessible from stairs at the east and west. With 7/7 magma below the whole thing and around 7 more units (total) on top, it's traversable most of the time... but trying is inadvisable, the magma sloshes around nicely.

I admit it's not the most deadly or FPS-friendly thing, causing flow, temperature and pathing calculations.
Still, I find it neat conceptually and literally. It's maintenance-free with no evaporation or magma loss, self-cleaning, and requires no crafted parts.

[Loud Whispers]

It's also a neat thing for designing a map you intend to produce seasonal flooding in; water-wise or magma. I use it for internal resevoir paths engineers can safely traverse (though unlike your design if water was on the top stairs whilst they were on it would go rather poorly for them) and mangrove-style raised huts. As for weaponising, I think it is rather amusing to have the down-stairs covered with hatches or corridors by doors concealing magma or water, which should allow for some good defence utility whilst avoiding pathing issues. Yet I don't understand how you could have sufficient magma to stave away evaporation whilst leaving the whole thing pathable, unless you have a pump cycle of some kind

[vjek]

I've built and used this design since 34.11 and since (at least) then, until now, in 47.05, there is an outstanding bug with respect to pathing on tiles over which magma has flowed, in this fashion.
So, it goes like this..
You fill up all the X and > tiles with magma 7/7, vaporize all the creatures in them via whatever manual/automation/pumping you have set up.  All good, works as expected.
Then, unless you save and reload, no dwarf will path over the > tiles, ever.  If you try to designate jobs/tasks on those tiles they will not treat them as pathable/walkable.
Checking the path to a depot will mark those tiles as un-pathable.
It also appears to stop events/invasions/snatchers from occurring, and if you manually spawn creatures outside your fort, they will not path inside.

Once you save and reload, everything works perfectly, until you flood them again.
It's a real shame the bug has existed for so long, as it's an extremely elegant form of very dwarf-ish defence. 
I've verified the bug still exists in 47.04 and 47.05, at the release of each.
iirc, the bug appears and is repeatable/reproduceable on both pure constructions (over a flat volcano embark, for example) as well as built just above the magma sea, out of existing/natural layers.

[Alastar]

@ Loud Whispers: I'm not sure I fully understand, I apologise if this is a non-sequitur. 1/7 fluid over permeable floor (down stairs, grates, bars) over 7/7 fluid should not evaporate. Side view:

X>11>>X
7777777

@ vjek: Looks like I'm very late to the party! My setup never completely fills the walkable tiles, just some passive wave action that shuffles height-1 magma around. I still noticed some pathing oddities: dwarves didn't path through it, wildlife wandered in and burned up. I had hoped that would work out in the players' favour (effective and 100% safe), but your experience makes me less optimistic.

[PatrikLundell]

Restricting pathing generally doesn't work reliably. You can set up pathing costs, but that only affects fortress members, with visitors and merchants being unaffected, and pathing costs won't prevent pathing but just reduces the likelihood of pathing over it (it doesn't do anything if the only path is through the area, for instance), and does absolutely nothing at all when it comes to controlling where a dorf stands when performing work on an adjacent tile (DF selects the tile to stand on without consulting either burrows or pathing costs).

[vjek]

@ vjek: Looks like I'm very late to the party! My setup never completely fills the walkable tiles, just some passive wave action that shuffles height-1 magma around. I still noticed some pathing oddities: dwarves didn't path through it, wildlife wandered in and burned up. I had hoped that would work out in the players' favour (effective and 100% safe), but your experience makes me less optimistic.

If what you're doing works for you, then there's no problem for you.
In my case, I have draining pressure plates in the hallway to start pumps that fill the entire hallway from floor to ceiling 7/7, and regardless of how brief or continuous the filling is, the bug still manifests, for me.  It's almost like DF is treating the drained stairway tiles as being full of magma, despite them being empty of magma.  Then when the save & reload happens, some other initialization 'check' is clearing those tiles and setting them pathable.
If you're doing something else, maybe you won't run into the problem?  All good then!  It sounds like, in your solution, typically the entire hallway isn't blocked or filled, and some other orthogonal/diagonal path is always available.  That may be preserving the pathing and not causing the bug I'm seeing.

The goal of my design was to make any creature(s) in that hallway fully submerged in magma to guarantee they would not survive. (unless they're a creature of a type that is immune to being submerged in magma, naturally)
Maybe someday the bug will be fixed, and more 'sealed' magma hallway designs will be created.