Most of the discussions at LW about Friendly AI seem to concern defining clear moral rules to program into AIs

I don't think this is true at all. In my understanding the general consensus is that it would not be efficacious to try imposing rules on something that is vastly smarter than you and capable of self-modification. You want it to want to be "friendly" so that it will not (intentionally) change itself away from "friendliness".

It's not a stretch to assume AIs will be capable of some form of self-modification

I agree. Isn't that the basis for AI-based singularitarianism?

and even if it's not their "intention" to do so, copying is never perfect,

I'm pretty sure Eliezer has put a lot of thought into the importance of goal-perserving modification and reproduction.