Bypassing a voltage regulator if input voltage is too low
There are buck switching converter chips designed to use a PMOS instead of a NMOS for the upper pass element. This allows 100% duty cycle, at the cost of a little efficiency loss due to the PMOS. Some switcher chips can also do 100% duty cycle with a NMOS, by using a charge pump. So you got a more efficient NMOS for the switching converter.
Converters capable of 100% duty cycle will just keep the top FET fully ON when Vin is too low for regulation (ie, 12V in, 12V out) so it's just the RdsON between input and output.
If you are going to design a switching converter anyway, and it looks like it since you put a schematic in your question, this could be a simpler solution to your problem...