A feat that changes the curse dice would clash with several magical items that manipulate the curse dice. Of course, it's trivially easy to just remove those items, but then you really have just changed your damage improvement option from item to feat.
More complicated would be revising the wording of these items to "stack" with the feat.
But maybe the simplest approach is to give a fixed damage bonus.
Dooming Curse
Prerequisite: Warlock, Warlock Curse
If you hit a cursed target with a Warlock attack power, you gain a +1 feat bonus to damage.
This bonus increases to +2 at 11th level and +3 at 21st level.
The bonus applies now almost every time the Warlock hits, not just when he uses his curse damage. But it's a feat bonus, so it is more limited and makes feats like Astral Fire irrelevant to the Warlock.
The armor proficiency is a considerable issue. It can be dealt with with the right stat selection. But it still costs you. But chainmal alone is probably also insufficient and most Star Pact Warlocks would want to jump to Scale or even Plate.
Maybe an even either approach would be to just allow the Star Pact Warlock to focus entirely on one ability score for attacks.
Star Channeler
Prerequiste: Warlock, Star Pact Warlock, Con15+
Benefit: When using a Warlock power that uses Charisma for attack and damage, you can use Constitution instead.
Hadars Armor
Prerequisite: Warlock, Star Pact Warlock, Con15+
You gain proficiency in Chainmail. When you gain the benefits of your Shadow Walk ability and wear heavy armor, an adjacent cursed enemy that hits you with a close or melee attack takes necrotic damage equal to your Charisma modifier.
It might also be nice to have some pact related abilities every time you curse a foe.
Infernal Curse
Prerequisite: Warlock, Infernal Pact
When you curse a foe, he takes a -2 penalty to melee and ranged attacks against you until the end of your next turn. This is a fear effect.
Radiating Curse
Prerequisite: Warlock, Star Pact
When you curse a foe, he loses all benefits from concealment and total concealment, and takes a -2 penalty to AC and Reflex defense until the end of your next turn.
Glamouring Curse
Prerequisite: Warlock, Star Pact
When you curse a foe, he takes a -2 penalty to all melee and ranged attacks against you until the end of your next turn. This is a charm effect.
Shrouding Curse
Prerequisite: Warlock, Dark Pact
When you curse your foe, all non-adjacent creatures gain concealment against him until the end of your next turn.
Ancient Curse
Prerequisite: Warlock, Vestige Pact
When you curse your foe and he makes an attack including you until the end of your next turn, he is weakened for that attack.