It's been a long time since I played OFP but I do have a suggestion.
However it depends on the answer to this question, can a missile or any other kind of projectile be deleted when it's in flight?
Maybe some kind of nearestobject code following a 'fired' eventhandler perhaps?
My suggestion is simply spawning explosions in a radius around a target, which can be done using any artillery script around with a little modification. You'd still see the missile being fired however. Just that it gets deleted very shortly after firing...in fact, near instantly. This way you don't have to mess with velocity of missiles in flight. This is just an idea and I have no idea if projectiles can be deleted.
If my memory serves me well, Mandoble played around with custom missiles and velocity stuff in his campaign Grunt ONE. Perhaps it's worth looking in there...it's also a very good campaign so worth a play if nothing else
EDIT: I just remembered that THobson played around with spawning projectiles in his Abandoned Armies mission. So by creating the right kind of projectile (such as a missile) at random points around a set position (a radius essentially), you would get the dispersion effect I think. That's where I would look. (How do I remember this stuff from 10 years+ ago