Just by coincidence, there was an article in the Los Angeles Daily News this morning about new rides and cheap ticket promotions for all our local theme parks. Alas only Shrek 3-D remains as this year's lone addition to USH.
Haven't been to this park in a while, but my understanding is that the Mummy attraction, like King Kong, is part of the back lot tram tour. In other words, you ride through a sound stage that has been set up to do Mummy effects. But I might be wrong because it's been a while...
Problem with USH is total lack of space to build anything significant, not unless the actual working studio part of the place gets the ax (and they've taken a lot of it away to give to the theme park over the years). It's not like Orlando where they built a park that has a few soundstages. Here, it's the other way around--studio first with its tour becoming so elaborate that it became a defacto theme park.
So there will probably never be a coaster here--just not enough space and the noise would be the death of all exterior shoots on the lot.
Hope this helps.