Would this be even vaguely conceivable? I'm thinking about the wavy red lines you get in Word and Firefox and such things when you type in an unrecognized word. I know that you can always copy your briefings, debriefings and messages into OpenOffice or something and check them. Then copy them back, but adding the functionality directly to FRED would make it much harder to forget that particular step. It would be far from perfect, of course, but it might help eliminate some of the more easily missed little errors and typoes that can be so immersion breaking when you notice them ingame.
Of course, I want to reiterate that this is purely a thought bubble. I have no idea how it might be implemented, or whether it would be viable or not. Just putting it out there.