R does not presently support Unicode and, unfortunately, it does not seem likely that it will in the near future. This makes it more difficult to use R for phonetics. There is however a way, admittedly painful and kludgy, to get Unicode characters into graphics generated by R. If you save the graphic in Postscript, you can edit the resulting file and insert Postscript to create the characters you need. Use some other character or characters as a placeholder in your R title or label.
The easiest way that I know to get a Postscript definition of a Unicode character is to use Yudit. Enter the character that you want in Yudit and print to a file. Then edit the resulting file. The file will consist of a prologue, most of which will be devoted to character definitions, followed by the actual image. What you want to do is identify the definition that corresponds to the character you need and cut it out.
You can identify the glyph that you want in the Postscript commands that actually generate the image. Yudit writes the image out in a funny order. The first line will be a comment giving the page number. The next line will be a "save" command. Then there will be a bunch of lines printing glyphs. The first of these contains the underscore for the file header. The second of these contains the character you want. The remaining lines contain the file header.
%%Page: 1 1 save gsave 64.000 751.000 translate Glyph0 grestore gsave 64.000 731.000 translate Glyph11 grestore gsave 64.000 764.000 translate Glyph1 grestoreSo in this case what you want is the definition of Glyph11. The definition will look like this:
/Glyph11 {
newpath
16.825 3.724 moveto
16.825 5.275 16.575 6.799 16.099 8.299 curveto
15.549 9.924 14.825 11.250 13.900 12.250 curveto
13.674 12.450 13.575 12.650 13.575 12.825 curveto
13.575 13.049 13.674 13.250 13.924 13.450 curveto
15.450 14.674 lineto
15.650 14.849 15.700 15.099 15.575 15.400 curveto
15.375 15.650 15.150 15.700 14.900 15.549 curveto
13.200 14.200 lineto
12.900 14.000 12.700 13.900 12.549 13.900 curveto
12.400 13.849 12.224 13.924 12.025 14.099 curveto
11.275 14.599 10.974 14.849 11.125 14.750 curveto
10.650 15.049 10.200 15.299 9.849 15.474 curveto
9.549 15.625 9.299 15.549 9.150 15.250 curveto
8.950 14.950 9.000 14.724 9.299 14.599 curveto
9.599 14.424 9.900 14.200 10.200 14.000 curveto
10.750 13.625 11.000 13.400 10.950 13.450 curveto
11.200 13.250 11.275 13.025 11.325 12.974 curveto
11.325 12.825 11.200 12.625 10.950 12.424 curveto
9.275 11.174 lineto
9.000 11.000 8.974 10.750 9.174 10.400 curveto
9.375 10.099 9.599 10.025 9.900 10.174 curveto
11.724 11.650 lineto
12.000 11.825 12.224 11.924 12.375 11.924 curveto
12.525 11.950 12.650 11.875 12.799 11.724 curveto
13.150 11.325 13.525 10.799 13.924 10.150 curveto
14.375 9.349 14.775 8.474 15.125 7.575 curveto
13.299 8.424 11.700 8.424 10.299 7.599 curveto
8.849 6.700 8.150 5.275 8.150 3.275 curveto
8.150 1.875 8.474 0.674 9.125 -0.275 curveto
9.849 -1.375 10.950 -1.950 12.424 -2.000 curveto
13.974 -1.950 15.125 -1.325 15.900 -0.125 curveto
16.500 0.875 16.825 2.150 16.825 3.724 curveto
closepath
12.549 -0.724 moveto
10.400 -0.724 9.349 0.549 9.349 3.125 curveto
9.349 5.650 10.400 6.924 12.549 6.924 curveto
14.549 6.924 15.575 5.650 15.575 3.125 curveto
15.575 0.549 14.549 -0.724 12.549 -0.724 curveto
closepath
fill
} bind def
It will begin with a name, such as "/Glyph11" here, and will end
with a "def" command.
Once you have identified the glyph you want and found its definition, save a copy of the definition and insert it into the Postscript file generated by R. Put it in the prologue, along with the other definitions. Then you will need to edit the title or whatever other string you wish to modify and insert a call to the glyph.
You should not have a hard time finding the relevant line in the Postscript file so long as there is a string of a few characters for which you can search. Here is the Postscript for a title.
435.34 542.73 (Spectrum of [dh] of English 'thy') .5 0 0 t
And here is the result of replacing the placeholder "dh" with the actual symbol:
435.34 542.73 (Spectrum of [) .5 0 0 t gsave 475.000 542.73 translate Glyph10 grestore 550.34 542.73 (] of English 'thy') .5 0 0 t
We have split the title into two parts, with the inserted symbol in the middle. In all three cases, the two numbers at the beginning (or after the gsave) are the x and y coordinates at which to place the bit of text. All three will normally have the same y value, but you will need to adjust the x values of the second and third lines to position the inserted character and what follows it correctly.