Invert matching lines, NUL-separated

If your comm supports non-text input (like GNU tools generally do), you can always swap NUL and nl (here with a shell supporting process substitution (have you got any plan for that in mksh btw?)):

comm -23 <(tr '\0\n' '\n\0' < file1) <(tr '\0\n' '\n\0' < file2) |
  tr '\0\n' '\n\0'

That's a common technique.