Exploiting the Faraday cage: They work, by having holes in the walls, smaller than the frequency/wavelength being contained. I should be able to emit a higher frequency, say 100GHz, through slots, or a grid, that at the same time, absorbs a lower frequency, say 20GHz, due to the wider wavelength.

Meaning I could plausibly do something like transmitting at high power, in the millimeter band, through "business end" facade slots, that are also absorbing radar pulses?