So a little late to the party, but I've been thinking about how to implement this (in particular, one of my players wants to do that lightsaber-y thing ways and means suggested), so here's what I'm thinking. I feel there should be different ways of adjudicating it based on what the targets are, stationary targets (walls, doors, a zone) or moving targets (enemies).
If the target is a stationary object, like a door or a wall, roll it more or less as suggested: Each exchange past the first, the Weapon rating of the spell just does direct damage--since the wall isn't "dodging" it doesn't make a difference, and the wizard doesn't have to concentrate on it to keep it on target.
For a whole-zone attack, however, I think the initial Discipline roll should still be used as an attack roll, on the basis of if you're filling a zone with magic fire for three turns, the fire shouldn't be any less deadly the second and third rounds. In this way, it kind of acts like a block, except it doesn't stop you from going into a zone, it just gives you a very good reason not to be in it. So if you put a Weapon:3 effect on a zone for two rounds, and rolled a 6 to control it initially, it'd be like an area-denial attack, where there's nothing stopping you from entering the zone, but you're going to be up against a tough attack if you try it.
As for single target effects, the way I'd do it is, for the wizard to land the attack, they'd have to keep rolling Discipline each turn to target as their attack, and if they do something else that turn, the attack simply doesn't hit anything (or it hits something they didn't intend as a compel). That'd keep things like the lightsaber idea and, say, Harry directing a fireball to hit several mooks in a row over several turns, consistent and keep you from abusing it to set up three or four constant attacks at once.
