Phonetic symbols (IPA) in R graphics

If all else fails (and hopefully someone who knows more about fonts and
such can give you a better suggestion so you don't even have to try
this) then look at the last example for the subplot function in the
TeachingDemos package.  This shows how you can insert images into a
plot, you could create an image file of the symbol you want to use and
then have subplot put it in the correct position. 

Hope this helps,