[R] [lattice] how to overlay a geographical map on a levelplot?

[Tom Roche]

r-help lattice adepts:

I have a question which is somewhat geospatial, so I posted to r-sig-geo
rather than here:

https://stat.ethz.ch/pipermail/r-sig-geo/2012-November/016757.html
> summary: How to overlay a geographical map on each panel in a lattice
> (or Trellis), e.g., of levelplot's? Note I am not inquiring about
> creating choropleth maps[,]

which Sarkar 2008 covers quite well (and which is supported by
latticeExtra::mapplot): I just want to appropriately overlay a
garden-variety boundary-line map on panel viewspaces defined by
longitude and latitude.

A relatively small, quite self-contained example follows the quote
above, in which I plot toy data in the sort of lattice layout I need ...
except that each panel lacks a map appropriate to the spatial domain. If
your competencies extend to that, your assistance would be appreciated.

TIA, Tom Roche <Tom_Roche at pobox.com>