Hi, I'm based in the UK and just came across this site and topic which I've also been researching. It might be a bit late, there's a UK company that's come up with a design for using tires filled with gravel as straw bale foundations which might be of interest. The design appears to have UK building regs approval, which is a big deal here! Drawings here:http://www.strawworks.co.uk/wp-content/uploads/2013/02/SD-Car-tyre-pillars-130226-05-29.pdf
Best wishes,
Mark