An update:
I cheated. I turned temperature off to get the dwarfs and their stuff underground and inside. Then I turned it back on. The world turned into a hellscape -- the river (and I mean, the *entire* river) boiled. Flames flew across the savannah, scouring it of all life and unlife -- for the undead deer were shown no mercy by the blazes.
Under the ground, the dwarves got by without the surface. Crops were planted, drinks were distilled, and mugs were crafted as the inferno raged on up above.
It rained, but the only effect was to have the water steam before it ever reached the ground. A similar effect can be seen at the mouth of the brook.
Occasionally, some new undead animals will show up. They'll appear around the southwest portion of the map, which is apparently cooler than the rest, but eventually they'll wander out of the safety zone, and *POOF*, charbroiled undead deer.
Unfortunately, despite the immense amount of mugs made in preparations for trading, the same is true of dwarven caravans. They appear on the map already aflame, leaving black trails of noxious smoke behind them like a pathway to hell. Out of the...six? dwarven caravans so far, one of them managed to push their burning mules hard enough to get them to the rampways leading underground, so that my dwarves could watch sadly as seeds, food (and most importantly) alcohol burn before their eyes, with nothing they can do about it. Even the metal bars ran like water in the hellish heat.
The rest of the stuff litters the cratered surface, having been forbidden to keep greedy dwarves from rushing outside to claim a xXSpider silk shoeXx only to combust six steps towards their goal.
The same fate awaits the immigrants; man, woman, child, none are spared the wrath of the heat. Most die out in the wastelands, where the fire picks their bones clean and the ungodly sun bleaches them to ash. Some become human torches, shambling flames that collapse inside the underground tunnel. All are remembered in the mausoleums far below the earth, given the coolness in death that they yearned for in life.
Glory be to Armok, five immigrants have actually made it inside more or less unscathed. This seems to be them appearing in the "cooler" quadrant of the map, where the undead animals roam. Construction is underway to open up more entrances here. Praise be, part of this area also contains an aquifer. Engineers are already drawing up plans to be able to flood the entrances with chilled underground water, dousing the tortured souls who manage to blindly claw their way underground through the pain of the fire...it is hoped that we may be able to save two or three this way, even if they are blind and bloodied and stunted for the rest of their existence...
(At first, I thought I would just turn temperature off when the immigrants appeared, and turn it back on when they were safely inside. Nope, they combust when it comes back on. Apparently, temperature is set on a creature to be equal to the area where they first step in?
Also, there's something incredibly amusing about watching an elven caravan take two steps onto the map, burst into flame, and then careen around in agony.)