Based on my experience with 16" Schwalbe Big Apples I'm inclined to think you could make something fairly useful using their two smaller sizes - 50-203 (12 x 2.00) & 50-254 (14 x 2.00) The 12" is rated for a 70 Kg load.

I'm pretty fussy about tires, and don't want to spend my ride paying close attention to road surfaces, so thats as small as I'd go. For tires that small, suspension would be a big plus, especially if it were part of the folding system.