Reverse polarity protection for battery powered application
With RDS of say 1 ohm you would have 50mV drop. This drop reduces the amount of energy you can drain from the battery a little, before the voltage drops too low, but otherwise it doesn't effect your system. Power loss in the RDS doesn't matter because you use a linear regulator so total power loss doesn't change.  So I'd say you can easily find good enough MOSFET even in SOT23 package.