I thought Jack's dad once said (in a vision to Locke, I think) that he "speaks for" Jacob.
I still think time-traveling Locke is Jacob. Jacob can't speak to his past self directly so he uses Dr. Shepherd.
It's possible that Jacob could speak to Locke if it was done so that Locke can't recognize himself but I think beyond that he has to use Jack's dad as a conduit
