Would I allow it: Yes. Warlocks are fun, but they are weaksauce. A warlock who got to throw his slightly weaker than Rogue Sneak Attack (but at far better range, and AoEs eventually) EB as iterative attacks, and perhaps even throw on Rapid Shot, would be able to better hang out with the big boys (read: the elitist clique of Cleric, Druid, and Wizard... as well as their less popular/powerful friends Psion, Sorc, Wilder...).
Would this be "balanced": An overused term that has no real meaning. The PHB 11 are not remotely "balanced" against each other and I am perfectly cool with that. If I put my head to the task of BOOM, I am sure I could do better with a scarcely optimized Wizard or Psion...