I was hoping to see a commercial coupler to figure out how they are
constructed, but so far have seen only one for XP radio, and it is not
clear what's up with it.

If by commercial couplers you mean the units for through glass antennas,
then the ones that I have taken apart consisted of a parallel tuned circuit
with the coax inner tapped a little way up the coil and the braid connected
to the cold end of the coil. The 'hot' end of the tuned circuit was
connected to the coupling plate. The antenna just had a coupling plate. Of
course it worked by virtue of the approx 1/2 wave whip having a high
impedance at the base, so the required value of coupling capacitance was not
extremely high.

73
Jeff