Glad to hear you are coming to Airventure. We enjoyed your article on the Lazair in the February Light Plane World.
http://www.eaa.org/lightplaneworld/issues/1002.html
You should definitely extend your visit. Rental of ultralights is very rare and probably not legal. Used prices are very low so you could buy, fly, and then resell later at not much loss. A single seat Phantom, Quicksilver Sport, or single seat Challenger flown as 103 would be ideal. A charming Aussie such as yourself would be welcome at any airfield.
If you hold any recognized pilot license you might be able to jump through some hoops and get endorsed for solo Sport pilot and then be able to rent fast 2-seat LSA's for $120 or more per hour.
A third option would be to plan to visit every every small fly-in and EAA chapter gathering in the areas you are visiting. Go for many rides (mostly free) in different aircraft at different places. Pay if you must. It's worth it to not have to maintain the airplane. Let us Yanks show you around the best sights. Use the EAA calendar to plan your itinerary.
--Dan Grunloh