Vinyl windows are not an option. This is in an historic district and
even if we wanted to (which we don't), we couldn't use them.

What you describe is exactly what I was thinking of doing - 'rout a
deeper rabbet', tho I think would still be hard to get double pane in
there.

Storm windows are also out for the same reason, but also, the concern
here is more cooling than heating (So Texas).

Thanks for the help.