By using the /opt/open-xchange/sbin/reloadconfiguration command line tool even the UI only plugins will be reloaded and should be available without restarting the server.

In a cluster environment the reload has to be executed on each backend node. Additionally you have to call touch-appsuite on the apache node. If you only run a single node setup the latter one is not required.

Please have a look at http://dev.ox.io/ for additional information about custom UI plugins.