The chainring steering idea is bomber — I also used it on the amphibious couch trike (
http://www.rat-patrol.org/RPOz/FArt/mitzie.html). I guess Ackermann steering geometry could probably be applied to that system if the chainrings were welded on slightly off-centre, but I haven't messed with that concept as yet. The only potential prob with this system is that the chainrings are easy enough to damage, but that hasn't been an issue for us yet.
Rack and pinion has the disadvantage of using non-bike parts, having inherent play in the steering, a limited turning circle and general gheyness.
Yeah, tippyness is an issue.
Master Gravity Esquire is a tad narrow for it's height and can roll at high speeds with a top-heavy load, as seen on our
Melbourne jaunt. Tha couch bike is as stable as anything on water... but on land, she's a bit tippy and has great potential for snapping human spines. The main prob with the couch bike is it's wheels are too big and therefore easy to pancake, so go the 22".