I haven't created a custom uni, because it seems to have to be tied to only Britechester or Foxbury.
But what I have done is create a new campus, in a different world that is tied to that uni. So, I have Foxbury-Oasis, which is based in Oasis Springs. I have only used the lots closest to the water, but I have made a Student Housing, Student Commons, a gym, and a bar/cafe that is my student hangout. There are other cheaper neighbourhoods nearby that I could change to more student accommodation if needed.
I figure that Foxbury is specialised in science, and what's a better scientific base than the desert? So I figured they would like a separate campus in a different location. Sometimes I have sims start off in the proper Foxbury campus, and then move to the desert for their remaining years. You miss out on the university events, but it does give a different feel to the uni experience. And then if you need to travel back to the Foxbury campus for, say graduation, then it kind of makes sense.
I also have a separate one for Britechester in Del Sol Valley, because of the drama/music degrees - I figure some sims would like to be near the lights and action.
