I recently encountered the same issue in trying to implement custom dimensions for Google Analytics. I wanted to add a dimension based on user roles. There is a way to do it without a GlideAjax call, though I must say this is about as far into the weeds as you can get from a platform perspective.
View the page source within ServiceNow, and about 60 lines down you'll find:
window.NOW.user = {};
window.NOW.user.preferences = [];
window.NOW.user.roles = 'admin';
window.NOW.user.id = '6816f79cc0a8016401c5a33be04be441';
window.NOW.user.departmentID = 'a581ab703710200044e0bfc8bcbe5de8';
window.NOW.user.firstName = 'System';
window.NOW.user.lastName = 'Administrator';
window.NOW.user.name = 'admin';
Within your UI Script, you can reference these variables, in particular the window.NOW.user.roles variable, to get the list of roles for the current user.
