I also said that " I know that the second Write() call which outputs to STDERR does output a valid pdf file.". I didn't think anybody would find it useful for me to post the contents of the entire pdf file. Here's the first few lines of output produced from the second Write():

%!PS-Adobe-3.0
%%Creator: (ImageMagick)
%%Title: (GLOB(0x92ed52c))
%%CreationDate: (Wed Jan 29 18:07:37 2003)

And here are the last few lines:

%%EndData
%%PageTrailer
%%Trailer
%%BoundingBox: 0 0 611 791
%%EOF