Not sure if my memory is correct, but wouldn't that just hasten global warming. The green house effect is when short wavelength radiation from the sun comes into the earth, and the surface of the earth would reflect the short wavelengths back as longer wavelengths, which get trapped in the atmosphere, thus the heat never leaves the earth.
Unless this satelite beaming down the microwaves is calibrated perfectly and is able to stay in sync with the man made lake, despite the rotation of the earth, this can turn ugly fast.
Also, I don't fully understand the Maser concept, so my theory may be completely irrelevent anyway...