The bomb may work like a missle: meaning, if it is "locked on" before firing, then it will guide itself towards the target; otherwise it just dumbly flies ahead (or down in the case of a bomb).
AFAIK, the only way to make the AI "lock on" to a target is with the dotarget / dofire commands. I think the "fire" command just makes them dumbly fire the weapon wherever they are pointed; although you might want to see what happens if you issue a "dotarget" command before the "fire" command.
If that doesn't work then the problem is, as you said, the way the pilot's AI makes him engage the target. There might be a way to get around his AI: try the disableAI "target" or "autotarget" command (on the pilot). If nothing else, perhaps disabling the "move" AI, and then guiding the plane via setvelocity commands.
If none of this works, then you are going to have to script your own guidance for the bomb, instead of just using OFP's built-in guidance system. There may be a script for something like this in the ed depot, or else you could hit up the Project: Mcar folks, who have plenty of experience with scripting guidance systems.