I’ve looked in the effect editor and the only options to control are “foreground” and “scaling.”
For the foreground there isn’t even a slider to adjust. For the “scaling” it basically controls the angle at which the wipe occurs. There aren’t any other controls present.
It has the border controls you mentioned for horizontal and vertical, but not for the diagonal. It seems kind of strange that it would leave out those controls for the diagonal wipe.
Dragzz