So you want something similar to ray casting, but you want the ray to have width. If any part of the ray overlaps with an object then you want the entire ray to be occluded. Is this correct?
You're right that the ray has 0 width. But depending on how you need your beam to work this might not be a problem.
I've seen a published game created in C3 do a wide laser using ray tracing, and I'm fairly sure they just cast a single ray down the middle. Most players won't notice. The narrower the laser the less noticeable the issue. Suppose that you render the actual beam by stretching a sprite from the source of the ray to the point of it's collision. That sprite has a rectangular collision polygon, so you can use that to check if any enemies touched the beam. Using this technique will not allow enemies to obstruct the beam, but you can check if the laser ( with width ) passed through them!
A more complex solution is to cast more than 1 ray for your laser. For example cast one ray for the left side and one for the right hand side. If one or both collided with an object then the ray was obstructed, the closest collision is the "real" one. This is faking the collision, so it's not exact. You might have issues with objects that are smaller than the width of the beam, or pointy collision shapes. You can tweak this by adding more ray casts within the laser.
A final alternative is to use the bullet behaviour. It's reflections don't have the exact angles like raycasting does, but it is quite close to correct and takes into account the shape of the objects collision polygon. You can use bullet stepping to prevent fast bullets from passing through objects erroneously.