I think making FoK/CT interact with BG won't really solve combat's abyssimal AoE. And here is why: Mastery.
Assassination: Fok spread poisons, poisons benefit from mastery, Assassination's aoe scales with every stat (agi, hit, exp, haste, crit, mastery)
Subtlety: CT is directly affected by sub's mastery. Sub's AoE benefits from agi, hit, exp, haste, crit, mastery.
Combat: MG doesn't proc off FoK or CT. Combat's AoE only scales from agi, hi, exp, haste, and crit. Any mastery on gear for a combat rogue is doing very little to your aoe rotation (and what little the mastery does is just from the extra energy from MG-CP procs on your autoattack target).
FoK and CT need to proc main gauche (but only your current combo point target should provide CP procs), or be changed in some way to allow combat's aoe rotation to benefit from mastery on gear.