This is what's happening. The character is attacking something in order to garner a benefit.
The benefit is 1d6+whatever damage. The normal way of doing this is to attack someone and deal damage to them with an attack roll. What the warlock wants to do is take some minor damage so that he doesn't have to make an attack roll to hit the opponent, at best. At worst, he's trying to up his damage by exploiting the DM's sense of coolness by 'minor action' pricking himself.
The Bag-o-Rats rule -most certainly- could apply here (the character is not a threat to himself, and it's to exploit a power, so even technically it follows it).
The -fact- is, he's trying to avoid making an attack roll to deal damage with a power not designed around that. If he wants to get the benefit of Hellish Rebuke he has to face -real- danger, not minor inconveniences. The idea of running in and trying to provoke OAs to up damage? Fine. The idea of forgoing attack rolls just to automaticly hit? Terrible.
Don't even entertain this.