R help - Jan 2011 - Displaying a PLS biplot under a 3d surface plot

All:

Anyone have an idea about how to plot a PLS biplot on the x-y plane under a 3-D
surface?  Preferably a 3-D surface such as one created with rgl.surface()
(meaning, I would love an interactive plot when all is said and done).

I have seen examples which combine a 3-d surface with contour plot (e.g.
http://stackoverflow.com/questions/1896419/plotting-a-3d-surface-plot-with-contour-map-overlay-using-r)
. . .  and I imagine I could construct the biplot 'by hand' via plot3d,
but it'd be awfully handy to be able to take the output of the biplot and
tuck it under the surface plot.

Thanks for your suggestions,
Ted