For posterity, I called Mr. Handyman, a franchised handyman service in
my area. He checked, and there was no electricity going to the post
wire when the switch was off. The gentleman first noted the absence of
GFCI on the circut for the post. Because there were some problems
putting a GFCI on at the panel, we installed it on the first outlet of
the series (the outlet was wired as a junction box!). He then dug up
the concrete plug and using a body length chisel, broke the concrete
apart and chiseled out the electrical cord where it had been cemented.
At night we covered the hole with plywood and rocks. We installed a
new post with a convenience outlet, filling the hole with cement only
below where the wire entered the post, and protecting the wire with a
piece of PVC pipe.