Good question. Since it would be two skis, I'd imagine I could sit on the back of one and pour into the other more steadily with a long tube attachment. The only problem, like Quinc mentioned, is chop and wind. We'd really have to find a flat day to do it on, and even still you never know how the weather could change. Planning to be picked up in Chicago possibly, but on the way there the only things I can possibly think of are either a backpack with a bladder and a long hose that I could fill the ski up with, or a separate gas tank with a huge fill hose.
Any ideas?