This is probably the best shot you have of getting that to work, from the BuffIncendiaryShellsSelf file.
instantAction
buffInstantActionType "ApplyBuffToTargetOnWeaponFired"
instantActionTriggerType "OnWeaponFired"
numWeaponClasses 2
weaponClassForWeaponPassive "AUTOCANNON"
weaponClassForWeaponPassive "MISSILE"
passiveWeaponEffectChance
Level:0 1.000000
Level:1 1.000000
Level:2 1.000000
buffType "BuffIncendiaryShellsTarget"
The bold is what you are going to want to keep (except changed to beam of course). The italics is what you are going to want to replace if possible, because then the buff is applied to your target instead of the Radiance. If not, you'll just have to change the buff to do extra damage much like incendiary shells. So I'd try something like this for the ability file.
entityType "Ability"
buffInstantActionType "ApplyBuffToSelf"
instantActionTriggerType "OnWeaponFired"
numWeaponClasses 1
weaponClassForWeaponPassive "BEAM(Or whatever it is listed as"
BuffType "Radiance damage increase buff"
...And the rest of the stuff found in the ability files (Energy Absorption is probably the best template for the rest).
