So, line 858 or so of xo-print.c (I may have messed up the line numbers very slightly with my debugging stuff) is

tmpfile is a char*, mkstemp returns a file handle. If, instead, you use mktemp, it gives you the file name, which is then what the later functions expect.

Kind of a silly mistake... not sure where it came from, but easy to fix. I can now export pdf files with text it them, and it'll get some good stress-testing over the next week with grading.