There is a native plant nursery here in Wisconsin that offers a mix for over septic fields. Even though these might not be the exact kinds of plants you are thinking of, they might offer some good ideas. It's called Prairie Nursery, and they have a website.

I have an identical environment and the former owner planted a lamiastrum. This is one time you really want an invasive plant. The neighbors told me she only planted a few plants and it took a couple of years to fill in but it is quite beautiful now. My lamiastrum has spikes of yellow flowers in the spring and variegated green and white downy foliage pretty much 12 months of the yr in zone 5. We have metal and black rubber edging around the leach field to contain it. I walk around 2 times a year with a bottle of RoundUp and squirt anybody who has jumped the fence. Its the easiest stuff I could imagine. The root system is only an inch or 2 deep but very tenacious.
Also I've planted lots of daffodils, mountain laurel and small rhodos in there under the trees and they've all done well.
Another option might be Houttuynia or chameleon plant if you want something taller. Check if it will take the shade. The lamiastrum does equally well in deep shade and in bright sun so I get a uniform cover through out the wooded leach field.

One more benefit is that we blow our leaves into the leach field every fall and by spring they are gone and the lamiastrum comes right up through them and grows on top.