My theory is the same, that a block wall wont hold up, long term, the poured concrete, with a high psi mixture, would last a lot longer, I am not sure that I would be as worried about wet clay pushing it as much as frost pushing and cracking it, maybe a little aggregate on the dirt side of it would keep either one, frost or shrink swell clay, from pushing too hard on it.
I like the looks of the one RO 40 shows in his pictures, If I understand that is similar to what you are wanting, except maybe a little wider and longer with a weir on each end. kinda like a lock and dam on the Mississippi River.
Good Luck! keep us posted with pictures.
