All you need to do is edit data/save/regionX/raw/objects/creature_standard.txt and add [SPEED:n] (where "n" is a suitably small number - 0 or 1 will make them lightning-fast) to the Dwarf creature entry, then load the savegame. When you're done, remove the token and they'll return to normal speed.
Hmm, so if the raws for the game itself are vanilla, and the raws for the save have been modded, the fort's save-game raws will override the install's?
Only the save's raws are used. Hence the full copy.
Wait, could we also mod Dwarves to fly, be able to live underwater, etc etc?
>_> Would being able to fly really mess up the FPS (all the pathing options)
I mean how awesome would it be to have a fort that considers flying? You could have huge spires like Coruscant or massive be-hive life forts with landing balconies etc etc.
There are enough bugs there that you probably don't want to try. At best, flying will let your dwarves go over walls and avoid stairs (that's all tame fliers seem to do). You should be able to avoid swimming issues by making them inate swimmers, though.