Airflow Performance runs a return line from the purge valve. It simply goes to a Tee fitting between the fuel filter and the fuel selector. No real need to add a complete return system or duplex selector valve.
That said, Todd Petersen of Petersen Aviation [holder of the auto fuel STC for 8.5 to 1 compression Lycoming engines] told me that having a fuel return path back to the fuel tank, will eliminate any fuel from vaporizing in the system when using auto fuel. He said that Lycoming's lack of this design feature was the reason that they were not able to get STCs for the fuel injected engines. The 8.5 to 1 carburetor engines gained the auto fuel STC just fine.
The fuel injected Continental engines, which have a return system, passed muster, easily. Just something else to consider.
Charlie Kuss