Was mucking around with WE and found something that could be useful here:
//generate -o black {v=((round(x/10+10*perlin(0,x/1,y/1,z/1,0.01,4,0.8))+round(z/10+10*perlin(0,x/1,y/1,z/1,0.01,4,0.8)))+(y))%20;(v>=-5)&&(v<5)}
Gonna try to make it into a script, because the command is a bit of a...