It's possible, the world map is just a bunch of superimposed bitmaps if you want to look at it that way. More complicated algorithms than "copy this series over there" will be more demanding on the CPU, i.e. slower. That shouldn't be a problem for anyone except those who generate a new world for every fortress - but if they don't use most of the world, they can as well generate smaller ones.
That's the thing, though--the world map does use mathematical generation patterns, which are basically fractals, and I'm sure that fractals would work fine if implemented; I'm just not sure that they
can be implemented at this point in the framework worldgen uses.
Worldgen determines topography and stone type/mineral content separately, topography determined with randomized fractals and geologic content determined based on another algorithm entirely, so to make a specific shape of a specific mineral combines the two in a way that, off the top of my head, I don't see any examples of in DF. All the other items, such as veins, are random pre-programmed features of a given stone layer, and exist outside of topography, which is why they can end and begin at strange places.
A tree that generates fractally would have to exist outside of a stone layer, given that it's not contained in a stone layer or soil layer, and would have effectively its own topography, by having a distinct shape. Currently, no stone or soil part of the game exists with its own premade shape independent of a stone layer, as in on top of or on the side of the layer. Toady would have to totally reprogram how stones work, and this might come hand-in-hand when we have true randomized veins based on geography rather than stone layer, or he would have to make petrified wood an entirely new object in the game, independent of stone, but otherwise, I can't see this being implemented where the trees are really made out of petrified wood as it currently is, a stone.