Browsing through an episode summary, lookies what I found... interceptors do work to a limited degree on Beams!
Source: http://www.midwinter.com/lurk/guide/054.html
The interceptors have two components, one that throws a ball of energy at an incoming weapons charge (physical or energy) and causes dissipation, and the other is a net-like energy web that reduces the severity, but does not deflect or absorb, beam type energy. This allows some time for maneuvers after beam contact.
Note that Major Ryan (He'll always be D-Day to my brother!) was very reticent to fire on the Clarkstown at all. Knowing that the Interceptors were down made his job all the more difficult. The rear facing beamn on the Alexander is similar in design to the front facers on the Clarkstown. When the C-town fired on the rotating section ofthe Alexander, it did not explode, as the interceptors were still active.
George Johnsen
CoProducer, B5
Source: http://www.midwinter.com/lurk/guide/054.html