Can the enthalpy of vaportization (latent heat of vaporization) be
"overcome" for saturated liquid water at 1000 kPa by creating a
pressure differential of 300 kPa via a pressure control valve?  I know
up to this point that energy must be put into the water to vaporize,
but can a change in pressure vaporize the water?

I remember in my high school chemistry class the teacher did a
demonstration.  She put water in a beaker and boiled it, not by
heating it but by putting it in a machine that created a vaccuum. 
When the air pressure inside was low enough, the water boiled at room
temperature.

As Ansel says, you can vaporize the water by reducing the pressure
below the vapor pressure at the water's temperature. You still need to
supply the heat of vaporization. If there is no source of heat, the
water will cool down as it vaporizes. This is how refrigerators work.

Jim

It sounds as if you are after what I call a 'heat knife'

Divide a set level of temperature into heat+ and heat- (cold)

I've been assured that it is not possible, which makes me suspect that it is.

We are basking in potential energy, kicking a rock of a mountain takes
little effort, but the avelanche at the bottom is pretty powerful.