Finding absolute viewport location in grid / lattice

I'm trying to do some post-plot manipulation of some lattice
graphics, in which I need to get the absolute viewport locations on
the plotting device. So for example:

library(lattice) print(xyplot(Petal.Length ~ Sepal.Length | Species,
iris, layout = c(2, 2))) trellis.focus('panel', 1, 1)

This shows the item I want to identify. I need to know, for example,
that the lower left corner of the highlighted panel is at (0.1, 0.1)
of the plotting device. Hopefully there's some grid shenanigans I can
use to do so.

To put the question into context (in case there's an alternative
way): I'm constructing PDF reports that are an amalgam of PDF
figures, constructed through LaTeX programmatically constructed in R,
and I'd like to be able to add tooltips to some of the data points on
the graphs. To do so, I'm overlaying a tikz (empty) image that uses
the pdfcomment package to add the tooltips. Hence I need to be able
to calculate where some of the points are on the screen so I can
calculate where to put the tooltip 'hit-zones'. This would be fairly
trivial using javascript / actionscript and mouseover hooks, but I'm
constrained by PDF.

Thanks, Jim Price. Strength in Numbers.

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