They have buyouts in one part of the park or another just about every other night in non-peak season. Diagon, New York, Hogsmeade and Marvel are the most common buyouts, with the occasional full park buyout.With the exception of groups and some special events though, I don't think Universal really goes after that "after hours" demo. With the exception of the busy seasons, I can't imagine there being enough demand to justify it. People can get EP just as easily.
They don’t need after hours parties since they make so much off the events.