A very interesting system indeed. Typical floatless carburetors have a standpipe in the bowl to establish the fuel level, with the exit of the standpipe connected to a scavenge pump. The question here is: what in the valving of the Willy's system acts to create a low (below atmospheric) pressure condition at the scavenge connection?

For the immediate problem that you are facing: closing throttle at high rpm, a barrel valve that both closes to an idle flow rate to the bowl, and, diverts excess fuel to the return line (like a high-speed bypass in an injection system) would be required. The key to your problem would seem to be in the connections between supply and return. What guidance does Willy's offer?