As to the hypothetical you posed (I know you love hypotheticals!) first, you know that Frances Wright was a big anti-slavery agitator, correct? But that at least through her books she was looking for resolution short of war....
Anyway:
Does this represent no injustice since enslaved people have no power to enter into a social agreement?
This kind of question takes us toward the logical conclusions, yes......
You are focusing on the "no power to enter in the agreement..." but it's probably the same thing to focus on the "unwilling" part, in that masters in slave societies throughout history are generally "unwilling" to enter into agreement to change that.
So when a slave murders a slaveholder, would justice be involved? I would say no, because there was no prior agreement by the slave that is violated, and even if somehow the slave had originally agreed (indentured servitude an example?) then Epicurus is still saying that the justice changes when the circumstances cease to be of benefit to the involved parties.
Another example might be the Greek slaves.... do I not recall they they where largely a conquered people? So presumably they too did not agree to become slaves, so there to a Helot (is that the name) killing his/her master would not be "unjust" or "just."
I want to reserve the right to revise these answers because I am answering them off the cuff, and no doubt there are tricky implications as with all hypotheticals, but I think the GENERAL point is that its very difficult to generalize about "an Epicurean" either northern or southern or modern or ancient. I think surely most people who admire Epicurus would agree that a state of freedom is far superior than a state of slavery, and therefore as to our family and friends we would certainly want freedom for them. As to ever-widening circles outward from that, no doubt the same generalization holds, but also I would think no doubt it weakens the further removed you are from the situation. Today you might pull out your guns and go attack anyone in your city who claimed the right to hold slaves, but we don't do the same think in those parts of the world today where the right to hold people in what is essentially slavery is also claimed even today.
I think a lot of this comes down to my prior comment that an Epicurean does not feel his emotions less than any other type of person - he feels them MORE deeply, and he's going to be willing to take action against what we detest MORE QUICKLY than a true Stoic would (thus the commentary in Sedley's "Ethics of Brutus and Cassius" that Brutus and Cassius did not bother trying to recruit Stoics into the conspiracy (or, at least, there were few Stoics involved was Sedley's point). So I think someone concerned about social injust need look no further than Cassius Longinus and Francis Wright as role models -- they can go hyper-Frances Wright and devote their entire lives to social reform, on the grounds of the pleasure that it gives them to do so - if they so choose. It's just that an Epicurean can't look to Plato's realm of ideas or to supernatural gods for "justification" for that decision.