Despite protestations to the contrary I've seen around, the Freespace *2* cutscenes I have are in MVE format, and FS_open seems perfectly able to play them. So the stuff about "FS_open can't handle MVEs" seems to be outdated. I suspect AVIs would use less space, though, so I'll look into replacing them anyway.
So, I downloaded the FS1 cutscenes in AVI format. On the advice of a webpage I found, I moved the FSPort files from the mail Freespace 2 folder to Freespace 2/fsport/, and added the cutscenes to that folder.
Now, the Freespace campaign no longer shows up on the Campaigns list in the main hall; I'm assuming that normally, you'd use the launcher to select FSPort with this configuration. However, OSX doesn't *have* a launcher yet.
I can't find the command line argument required to specify a mod directly to FS_open. Could you tell me what that should be?