Yes that is a legal move, covered in
Rule 7. If you are alone in a hog, you may exit and attack an enemy hog if a) it is a rocket that is approaching you, or b) a chain hog firing at you.
So if a chain hog isn't firing yet, you have a decent chance at driving past without dying. If they start firing, you have the option to defend yourself. But with a rocket you don't have much chance to wait and see if they notice you and fire, so you are allowed to get out of your hog and attack them. Only of course if the enemy has a gunner.
In our scrims, we can only exit to attack if fired upon, even if it's a rocket. In public servers, we cannnot exit to attack at all. We can hide or evade. So no, that its not legal for BFM players to do.
I know BFM players cannot grenade a hog if there are enough for everybody to drive, even in self defense
Anyone, including BFM members, can nade a
loaded hog (with gunner) in self defense.
BFMs cannot nade a
lone hog (no gunner) unless there are no other hogs available. Within reason, guests can nade any nearby hog, even if it contains an lone enemy driver, as long as they take that hog and drive off with it. But BFMs would have to check for other hogs and if there were any available, they'd have to take one of those.