Can priorities be constructed diagonally?
I'm not at home at the moment, so can't build a "real" example, but the answer is Yes, you can build a priority junction diagonally.
This quickly thrown together example should demonstrate how:
Basically, you must ensure the track crossing the junction and prioritised line cannot be accessed by any train - so here the sections where the control track touches the diagonal track are all attached "against the flow" so that they are impossible to enter.
Once that's done the the signal placement is effectively the same just with the roles of the tracks swapped.