Very quick, very cursory look at the code suggests that it might be so simple as to alter this to something higher than 10:
From missioncmdbrief.h: #define CMD_BRIEF_STAGES_MAX 10
Tell you what, if noone else gets around to it before tomorrow, I'll grab the latest stable code tree (what I have is somewhat outdated by now) and post a build here for you to test with that bumped upwards a bit. There might be more to it, but it'll be 500 times faster to just try it and see than to dig deeper at this point, so it's worth a shot. And FRED references the same define, so that should be taken care of as well as far as I can tell.
[Edit] Having a bit of a problem getting FRED to compile, but I do have a build of FS2 with the bumped limit ready in case you want to use notepad hacks to try it out. Let me know if you want it anyway.