Mike, I can't help much, but I think if you did some reading on "swimming pond natural filtration" you would find many ideas for filtering your water with plants, gravel, fountains, and such. I am sure this means $$$, but it would certainly be an educational read and could yield some possible solutions for you. I can't help but think that a natural filtration system would be better designed if the water was pumped and allowed to flow back in rather than relying on periodic in-flow through the system. Slower constant flow can be filtered much better (and controlled off an on) compared to the high flows of an in-rush.
I do not understand what happens when the "natural filter" loads up with what it's filtering out...I guess you have to excavate and start over. It's not like you can pull the filter and replace it with one from the hardware store.
There's your $0.02 back I borrowed!
