We are heading to Disney in Sept and I was also trying to find a more convenient way to deal with the car seat situation. Since FL law says my 4 year old can be in a seat belt, I thought about that option. I also looked into a portable car seat that folds up and can be carried in your suitcase. Then I read a story about a mom who had put her child in a seat that was not really the right fit and they had a head on collision. The child is now paralyzed. At that point I decided that we will bring our kids full carseats with us, even if it may be inconvenient to take them on the plane, etc.
I couldnt live with myself if anything ever happened that may have been able to be prevented if we had the kids normal car seats.
As for renting mini vans, we found that most places will not guarantee a van with built in seats. All the companies we called said they carried two different types of minivans and one had carseats, the other didnt. So you wont know if you are going to get one until you get down there.