Answering my own question:

http://pages.cs.wisc.edu/~ghost/doc/AFPL/6.50/Language.htm#Transparency

For some reason this was a bit difficult to find.
I just needed to adjust the code:

   { gsave
       [ /ca .5 /SetTransparency pdfmark
       .5 .setopacityalpha true .settextknockout
       ...

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