I too would like to know if this is possible.

The Beam blast ability might be the most accurate ability, though it is not possible with normal weapons.

I know you can via abilites passive or active. Finest hour or Cluster missles for examples.

That would kind of go against the idea of recharge, especially if it is for balance reasons.

To make weapons AoE make a passive ability and the buff will be an instantAction where the apply is ApplyBuffToTargetOnWeaponFired and the trigger will be OnWeaponFired. The next buff is ApplyBuffToTargetsInRadius and the last buff will be a DoDamage buff.

I think the best reference are the buffs for AbilityChargedMissiles (this is not a passive ability but an activate/deactivate ability) you will only need to copy the InstantActions of the buffs.

I haven't looked at the buff system yet but I intend to fairly soon to rig the Heavy Plasma Torpedo + several large-scale beam weapons. This sounds fairly complex, but hopefully I can manage it.

It sounds like from the buffs you are naming that the AoE damage is applied instantly upon firing? That may not be desirable for the Plasma Torpedo in particular since it's a slow-moving projectile with a range of about 90k+ pending balancing (gravity wells are much larger in this mod than in vanilla). But that's just what I gather from the names - once I get my new batch of ships converted to mesh format I'll try this out.