How would that achieve what the other poster is saying, which is to heat the insides more than the surface? Wouldn't that just heat both the insides and the surface more evenly?
Even if this was the case - the insides and surface of the human body have rather different characteristics, with the inside being more sensitive to, well, just about everything than the surface.
But do longer wavelengths act on water in the same way as microwave?
They act on systems of distributed induction and capacitance which are resonant to that longer wavelength.
Longer wavelengths require huge antennas to concentrate the energy at a distance. It would be probably very difficult to perform this attack. I also think that such energy density with relatively low frequency (~3 GHz) would disturb operation of computers, phones and other electronics, so it would be pretty suspicious.