But in emergency/expedient situations, it would be really handy if one
could pass at HF, say, 100 watts of RF energy through a window (I
visualize some foil strips and an inductor or two to "tune out" the
capacitance) to an external antenna.

What I have done is plan ahead by drilling two small holes in the glass
and plugging them with nylon screws when not in use. That works best
on the first floor. :-)
===============================
Might be OK for single - ,but NOT for double glazed windows.

A solution for HF 'might' be an inductive coupler/matching unit , with
half of it including an inductor on the outside with the other inductor
and 'tuning bits' inside .Both inductors 'in-line' for best possible
coupling. If an outside variable capacitor is to be used ,its shaft
could possibly be magnetically coupled with a manipulator fitted inside
,using strong magnets .
Home-brewing Ham Radio equipment is and remains an interesting
ideas-provoking activity .

Frank GM0CSZ / KN6WH