Some aircraft have a return fuel line
Do you know which ones?
There are definitely some installations where fuel is returned to the tank(s) but then I wonder how this is dealt with when installing a fuel totaliser. I have seen some STCs for those, for Lycos, and none make a provision for any of this.
The most common Lyco engine fuel system configuration has no bypass whatever. When the fuel servo is shut (closed throttle) you cannot pump any fuel into it. So both the engine driven pump and the electric pump are pumps with a
built-in output pressure limit (either implicit, as in a diaphragm pump, or explicit, as in a gear pump with a spring loaded bypass valve).