It's available after Quincy. I ran the Bonus.exe file available here but it didn't modify my Bos.cfg file like it is supposedly meant to. This file is in the Core sub-directory. I had to open it with Notepad and edit the line:-
{misc.bonusMission} = {false} to {misc.bonusMission} = {true}.
I thought I'd missed the boat on it when it was not there after Quincy but I edited that .cfg file whilst playing Mardin and bingo, it was there as an option the next time.
