[ssl-3]

With bias power, I think an electrostatic loudspeaker turns into a condenser microphone (a thing that provides varying capacitance in response to changes in pressure).

I don't think that electrostatic loudspeakers all require bias power, so it's not quite as simple as using a dynamic loudspeaker backwards is.

It is a neat idea, though. A big, flat-panel microphone would be interesting to play with.

[robotresearcher]

You can use a window or any large panel as a microphone without even touching it by observing its vibrations.

You can bounce a laser off it, or even go fully passive using a camera with some sensitivity tricks: I recall a paper that reconstructed a remote conversation by watching a houseplant through a window.

[ssl-3]

Of course.

Everything is a microphone if you're brave enough.