Accessing ggplot aesthetics using

Continuing the discussion from Accessing ggplot's inherited data object from a layer:

I wrote a function to annotate my plots manually. This draws a crosshatch at a target point and writes out the target value:

annopt <- function(x, y) { 
  list(annotate("point", x=x, y=y, size=5, shape=13),
       annotate("text", x=-Inf, y=Inf, label=paste0(x,",",y), hjust=-0.2, vjust=2))

To annotate an plot I can tack this onto the end of a ggplot object:

p <- ggplot(iris, aes(x=Sepal.Length, y= Petal.Length, color=Species)) +
p + annopt(7,5.5)

I have a table of target values and they get used in numerous plots. I wish to write a function that inherits the x and y aesthetics from the ggplot object, looks up target values in a specification table, and returns a list of annotations like in the example above: gs_annopt(spectable, specname).

p + gs_annopt(myspecs, target)

Do I have to dig into ggplot_build to extract the x and y strings or is there a more elegant solution?

I should clarify that I'm not asking how to lookup the value in a spec table. I've written code to do that. I'm trying to understand how to get the parameter name from the ggplot object.