Just my comment to "but I suspect the overall ride experience to be a bit bouncy and flexy".

Kwiggle is a new movement for a flexy human being. Humans have been accustomed to putting their natural flexibility into a rigid posture on a bicycle for 200 years.

If you are thinking the bicycle from the anatomically view, you should invent a bicycle, that could be ridden upright with your flexible natural movement similar to the walking.

This was the first idea of the Kwiggle. And this is an opportunity to get back to your natural flexibility. After a little get used to, Kwiggle will adapt to your movement and not the other way round.