can you open the file in Adobe Illustrator or Photoshop to add the text? Then you can just export the file again to an .eps file.

Or, are you familiar with Tex/LaTeX enough to take your image, input it into a tex script to crop just the image, add the figure # text to the bottom and then make THAT into a postscript file?

I've got ready made scripts at work (probably on my desktop computer here at home too) that I can send you that should theoretically work... as it's GRL, don't they want a TeX/LaTeX file to be submitted?