I don’t think things have changed in recent versions, so see this thread which might help you. http://community.thomsonreuters.com/t5/EndNote-Styles-Filters-and/Style-needed-Format-Mey09/m-p/7298 but it looks like your requirements are a bit different, so Masaaki’s script may not work for you.
see also http://community.thomsonreuters.com/t5/EndNote-How-To/Citation-with-style-ABCD05-with-EndNote-X2/m-p/2742/highlight/true#M881 where a workaround is suggested where you manually create each of the short-hand bracketed reference.