Screenshots of what you're doing.

First, I need to find a way to turn on and off particular light entities to imitate lighting from the cannon beam. relay_activate and relay_deactivate don't seem to have effect on this. Second, some means to interact with shader parameters from within the game must be found (e.g. for changing textures or applying special effects to them) for the duration of firing stage.
Or maybe it's better to remove ability to launch it manually and set the cannon to fire on certain intervals instead? Although that wouldn't be as much fun as first variant. Those who have played Half-Life Deathmatch surely remember the crossfire map. Launching firing sequence immediately turned gameplay on 180 degrees, as all players from every part of the map began to rush towards bunker with losers getting smashed in an airstrike. I'd prefer that kind of gameplay here.

This needs more attention. As for myself, I'd like to see mapQC, or at least ability to add some sort of extended scripting to the map. (Yes, I know the crossfire map. It was fun.) As a matter of fact, this should be done in singleplayer, for various details and cutscenes. I don't know if it's going to get done in the main game, or will the sp team have to do this themselves.

Or maybe CSQC operating on a map with some hack triggers on it? The firing sequence would be triggered by a button, but it could be intercepted by csqc function...