While you don't need the specific rates, but rather just GC, history of LIBOR is still easier to obtain. However, I don't think it's the right approximation, especially in this day and age. In fact, at the moment, there's a LOT of action occurring, which makes things even more dicey.I guess one of my unspoken approximations was repo rate = LIBOR because it's much easier to get history of LIBOR rates than history of repo rates on specific bonds. And you can say that was a poor approximation to make (though not as bad pre 2007).
And yes, this discussion is pointless since I've already given him what he actually wanted!
GAT
And yeah, I think you've given him what he needs.