Can pedal-based power meters be used as flat pedals?
All road clipless pedal designed for shoes with cleats cannot be ridden with normal walking shoes - they don't have a flat platform for a shoe's sole to rest on.

Power meter pedals are probably all clipless. You might find some MTB pedals that have both a SPD cleat systems and flat platforms.

The easiest route is to get a left crank arm meter which woudl allow you to use whatever pedals you like.