One concern is that there's no direct feedback to the endorser, so he or she would have no sense of the relative importance of their endorsement to keeping the discussions rolling along. With visions of the Stack Overflow police, how high a bar should a comment have to pass? Although maybe that's a positive -- after all, we no longer directly see comment scores, only the derivative effect of thread reordering.
So, have a new meta-karma that is equal to the mods given to the posts you endorsed? (An alternative would be giving karma directly, or some fraction of karma).
Perhaps that could work, although it goes a bit down the road toward heavier and more explicit mechanics, as at Slashdot and Stack Overflow. In another comment somewhere in this thread, pg mentions that he'd like to keep it as simple as possible. Unspoken is the "...but no simpler" part.
It's fun to consider that a dynamic system could provide someone on the back end (or a smart algorithm) with a variable nozzle controlling comment flow.