Different labels for subsets of points in a PCA or RDA biplot

I've tried a few things both with prcomp(), and rda() and its friends
in vegan (including biplot.rda and ordiplot), but can't find a
solution. I'd like to associate subsets of the points in a resulting
biplot ("sites" in the rda object) with different plotting colors/text
styles to emphasize certain sets of points. I can't figure out how to
keep the arrows (for "species" in the rda object) and then pass sets
of points and their labels separately. A subproblem is finding out how
to associate a vector of text labels with the coordinates in the rda
object. I had no problem doing this with 'xlabs' in biplot.prcomp(),
but can't get 'labels' to recognize such in ordiplot functions.

Dave Hewitt