I think I found a simple solution, although it requires some tinkering to 
find the x,y coordinates of the plot region to place the text...

barplot ( )
text(x= 2.9, y = 0.43, srt=90, labels = "H", cex = 1.5, col="blue")  #srt 
rotates the "H" character, so that it resembles an error bar
text(x=3.5, y=0.432,  labels = "95% C.I.", cex=1.1)
rect (2.62,.41,3.9,.45)    #draws box around text

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