Retrofitting an outhouse into a treebog

At a property I am co-managing, there is an outhouse that is basically a roof, three walls and a door around a toilet seat on a stand on top of a 1 meter long .5 meter wide concrete tube vertically submerged in the ground. The tube is full of human waste, mostly liquid. It doesn’t drain into the ground because the subsoil is heavy clay. What trees could I plant around the outhouse that would reach roots in under the concrete tube and suck out the material, including liquid? Would willow work? Poplar, birch? Would comfrey do anything? How would I go about retrofitting such a situation into a treebog?

Consider having the toilet pumped out for a start and then redesigning it.

...  I am reading several reports of don't plant them near septic tank drain fields as they will rip everything up - this makes me think they will make excellent poop beasts!

- Paul Weaton