Seeing Geert sent my his 2x2x4 article, I decided I'd better show how mine is coming along. Basically, it does seem to work, the cubies cannot fall out and it looks like it's a tangable and simple enough method.
1. This is the modified Oddzon 2x2x2 piece. You can see that I have cut slits in the left and top cubie walls. Also, above this, a rounded groove has been cut through the roof. This allows the neck of the new layer piece to reach through and hold underneath the surface of the main piece.
2. This is my ugly test layer piece. The bulk is made entirely out of milliput, as is the extended neck part. The lip is a piece of phone card superglued to the top. This will be slightly thickened and properly made later on. For now, it's functional.
3. Ok, here is the piece is an assembled grip-test. At a turn of this angle, it is the most exposed, yet impossible to fall out.
I'll keep working on it over the next few days and run a test mold and casted pieces and see what happens. Still, without molding pieces, others can simply craft each new-layer piece by hand easily enough.