Haven't tried this yet, but couldn't you reduce the foaming by inserting a large-mesh screen (fitted to the size and shape of the fuel tank tube) into the filler tube? I'm thinking about a 1/16" mesh. It seems that if foam bubbles won't fit through the mesh they will break when the they make contact with the screen and the bubbles will pop causing the residue to head back down the tube, breaking up more bubbles as it collides with them. When I was a kid I used to lay my finger across the top of a glass to dampen the foam from soft drinks as you pour it over the ice. They would pop as they hit my finger, condense on my finger then drip back into the glass.