I proposed to my wife at universal and called guest relations and spoke with them while I was there.
You can't do anything that could disrupt a show, this includes rides and shows so that also includes things like the wand shop. They did say that if there was a favorite character they could have then present but that is pretty much the extent. I also asked if I could have one of the park photographers in a certain spot at a certain time, unfortunately universal doesn't control over them and was told to ask one of them at entry if they could handle my request.
I had 5 places in mind depending on where we were at the time: navigators Club bridge (can't be backstage), the rock area near the mythos bridge, alcatraz area on the water near bruce, battery park, and on the rock it.
I went with Bruce since we were both huge jaws fans, the area has a great view, generally had a photographer there, and had a liquid courage dispenser right there. We didn't end up getting a picture but we got a drawing from the caricature artist right next to it, which led to part of the theme of our wedding.
My biggest word of advice is to make sure that you have a safe and secure place to hold the ring, there was no bigger fear than riding mummy and MiB with it in my pocket.
I would definitely still call guest services to see what they can help with.