I've just been adding students to new courses by name and got annoyed at waiting for the server to search all the time, so I've written a small bit of javascript to automatically select the first matching name starting with the search string as the user types. This allows the first name to be partially typed and 'Add' to be clicked without scrolling or waiting for a smaller list.

Patch (roles_assign_html.txt) is against Moodle 1.9.2.+ (20080901) - apply it to /admin/roles/assign.html and add search.js to the /admin/roles directory. Patched assign.html is included too if you want to just overwrite the file.