I understand how you think. Problem is there is a lot of info floating around out there but to find it all can take a long time.

I found several looking trough the old posts last night but one had racks that was too wide for a bus rack (maybe that could have been changed),
and several do not have the seat set back as far as you want.

Personaly I do not like the forward BB style becouse I feel i have no strenght for hills riding in that position.

When I start on a project of heavily modifying stuff (like making a homemade longtail) I often make a rough sketch first- Make something that does not involve much work but can give me a feeling of what the finished product would be: What about starting off with something 20" (like a R20 or similar) and fit a BMX setback seatpost? That could give you a feeling of how much setback (seat vs BB) you want, and can also give you an idea of using a "shopper type frame" w setback seatpost is an option at all.

I remember several yrs ago a bike was posted several times in the forums: A BMX (right size and strenght) with two long pipes clamped to the frame, one on each side. Gave exactely the same effect as the one you linked to but with two pipes instead of one. Made a stable front and rear rack to clamp platform or box to. Could be welded and made more neat looking. An extra toptube added to a nonfolding R20 or a steel minivelo (a lot of them being discussed in the BF foldingbike forum) both options with a setback seatpost could be a start (like a sketch) or could be good enough if you are lucky.

Did you see sixty`s old R20 converted to a touring bike with longer wheelbase? I think he chopped off the seat and chainstays of it and welded (or brazed) the rear part of a full size bike to the R20. This way he got a longer wheelbase and less heel strike when using a "normal" rear bikerack and full size panniers. In your case I was mostly thinking of the better balance this would bring to the bike with a setback seatpost.

We need a sticky with links to good looking 20" cargo bikes so that they are easyer to find!

I think your project is interesting but it is a bit advanged for the regular utilityrider and is a mix of stuff you`ll also find in the foldingbike forum, the Alt bike forum and the framebuilders forum.

Keep us updated!