The BMS file can only handle 255 characters of information for the briefing and unfortunately that's it.
Now the mission that come with the game don't have that limitation and most likely will not even have any information in the briefing part of the BMS file. Novalogic has hard coded those briefings into the game.
There is a BRIEF.BIN file that is in the PFF file. The briefings are mapped to a BMS file names that the game uses. So in theory, you can create a BMS file and name the same as one that is in the game. When you go to play it, it should give you the briefing information for that mission.
The only way to change that is to go in and change the text in the BREIF.BIN file. The only problem with that is that everyone would have to download and repack the BRIEF.BIN back into their PFF for them to see the information.