Actually, the best time to board an enemy lev is if she has no way to move, save up or down. However, there's even a limit there as a defensive tactic. Too high, and you'll keel over from lack of oxygen. Too low, and you'll practically be grounding the ship, and making boarding far more easy.
So, if someone is keen on taking a Live Lev... well... alive, then the first thing to do would be to take out those motors. Then, when she's dead in the air, you move into position to hook up.
Not wise, but better than lassoing a bucking bronco.
Y'know, I can imagine some of the smaller countries working up a fleet of 'predator' ships to steal whatever they can from the more successful navies, and not necessarily as a pirate gimic.