As it is now, JQuery UI does not recognize the library. To that end, it should be as simple as creating a symlink from /sites/all/modules/jquery_ui to /sites/all/libraries/jquery.ui thus:
$ cd ~/firstsite.com/sites/all/modules/jquery_ui $ ln -s ../../libraries/jquery.ui jquery.ui $ cd ~/firstsite.com/sites/secondsite $ drush pm-enable jquery_ui Module jquery_ui don't meet the requirements to be enabled. [error] There were no projects that could be enabled. [ok] The jQuery UI plugin is missing. Download and extract it to your [error] jquery_ui module directory. (Currently using jQuery UI Not found) An error occurred at function : drush_pm_enable [error] $ cd ../firstsite.com $ drush pm-enable jquery_ui jquery_ui is already enabled. [ok] There were no projects that could be enabled. [ok]
This method works fine for the main site, but is not recognized by other sites in a multi-site install. By drush or by drupal interface, the jquery.ui library is not recognized or found by satellite sites in the install. For instance /sites/secondsite.com would not be able to take advantage of jquery.ui with the symlink.
I wish I knew enough to be more useful, or I'd have a go at it myself. All I am able to do right now is point this out in hopes that some very-informed person will put this on the front burner along with a fix for the missing library integration for jquery_ui: http://drupal.org/node/489140