Some more info/synthesis of info about the build:
We're not interested in using cinder blocks, urbanite, etc. because of the toxicity issues (use of fly ash, etc).
We're also not interested in using plastic (which probably excludes earthbags).
The earthen floors I have built before just had timbers sills to retain them, rather than stone or concrete, but yes, they need some kind of retaining wall, and in the humid subtropics where this build is going to occur, wood is out.
We are leaning toward concrete (we are fine with making the forms and pouring concrete), but are looking for alternatives due to the toxicity issues and high embodied energy in Portland cement concrete. We would have to buy stone and have no experience laying it, so it's probably out, although I might consider slip form rock with lime mortar, but not sure about the structural integrity without cement mortar. Bricks seem to have at least as much of a toxicity issue as concrete (although a lower embodied energy), but we probably won't go that direction since commercially available and/or salvaged bricks seem likely to present more of a toxicity issue than the concrete.
I'm guessing that the ground-floor-wall interface will look something like: gravel subfloor filling most of the space (probably 12-18") between grade and the top of the stem wall, covered by a few inches of earthen floor to bring it up even with the top of the stem wall (on which rests the bottom plate of the stick framing). It's not very elegant, but it leaves a foot+ of space between any wood and the earth, which is what I want, although it's not really inspectable for termites if the space is full of gravel. However, I can't figure out how to have it be hollow and have an earthen floor, and we really want the thermal conductivity of the earthen floor for cooling purposes. There will probably also be earth tubes for cooling running through the subfloor and up into the living space.