I know this reply is too late, but for others to see and comment on i'll post anyway.
I, too, had this same question (albeit I worked my *ss off on my brief) last spring. There's nothing stopping you from using any authority that you had not discussed in your brief. It might be helpful during your argument to acknowledge that such authority is not in your brief in order to give the court the opportunity to take special note. I find that the point of oral argument isn't to reiterate what the judges/justices can already read, but rather to converse with them as to why you have the better position. If pulling from other non-tabled sources helps you do that, go for it.