Mark, you asked, "roger, in your first post, you say that the "purlin could be severly weakened" Did you mean the principal rafter?"

There would be two basic schemes for the spline, first the flat of the spline square to shaft of the rafter and second flat parallel to the shaft of the rafter through the neutral center. The former would be unthinkable due to damage to the rafter and the latter safe to the rafter but would create a weak purlin if the mortise is open side to side.