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?

This is something that nearly all (non-industrial) microwave ovens do.. They contain the microwave band magnetron energy (~2Ghz), while allowing visible light to pass through (400-790THz), with a conductive-enough mesh door. At ~15cm wide, the "micro"waves are far too large, to pass through.