I'm not really familiar enough with how panes are working in ubercart (I've kind of written my own checkout process over the top of ubercart in order to get the multipage thing going), but can we just render panes on multiple pages, take the submission data and put it into a "dummy" order object until submission of the last page, upon which a new order is created?
Or is everything tied up too much with Javascript currently?

The approach of having multiple pages could also bring up a few issues with concurrency of cart and order (at which point do the contents of the cart join the order?)