Great vid, good to see I'm not the only one with these crazy ideas.
Not a good idea to use a standard kitesurf kite as they are too efficient, especially for a direct downwind run. You need a kite that sits deep in the window. The F-One SUP kite does this and works well but were simply a lot more money than most would want to spend.
For direct downwind runs you could use a single line kite and attach it just in front of the stance area somewhere, this would also free you up to use a paddle.
Mikes idea with a trainer is a good option. Probably good to stick with short lines he could launch it, especially when it is open cell and would sink if he crashed it. Only problem is such short lines kill all the power.
Another option is a trainer that water relaunches. This video shows the concept but looks like it was filmed in very light wind-