I guess I'm not convinced that the critism is true.
Almost all of the heat transfer texts I own include microwave energy implicitly in the use of the Stefan-Boltzman sigma*T4 relationship, which includes microwave radiation. For the simple body to body radiative transfer, microwaves are included.
However, we should note that most of the heliostats were developed using mirror arrays with limited apertures. These limited apertures will not properly reflect nor focus wavelengths comparable to the aperture dimension efficiently. Thus, it might well be that the issue is an optical and not thermal radiation problem.
TTFN