I didn't even think of it when I wrote the original reply, but if the locations share power grid wires internally you can always g.hn over that copper to get the connection through the wall.
Someone else suggested Ruckus in another subthread here. If they support wireless downlinks, that may be a good option.
EDIT: If it's known that (quality) power line stuff does not cause interference, it does become a good option.
i consider it somewhere in between wifi and an ethernet cable in terms of quality.