i have some dwarf
apple trees on my drainfield edges, and I also have some honeysuckes, cotinus, lilacs and other shrubs..but they are along the sloped sides of the 4' raised drainfield..not up on top of the drainfield itself (that is lawn)..
you can see photos in my blog (address)..but most of the trees are much too small to see as they were babies 2 years ago
I planted the entire sloped sides into mixed beds..and it seems to work out quite well, but the top circle is lawn.
i have dug up and attempted to move (after my housefire) dwarf fruit trees, it is amazing how small their root systems are as adults..but it depends on the tree itself ..
generally most trees root systems do reach out a little wider than the top
canopy, so you can figure if you have a canopy of 20' that you probably shoul have the tree about 20' away from your pipes..(half of 20 ' canopy being 10 ')