Dialog to edit template definitions is defined in ckeditor/plugins/mwtemplate/dialogs/teplate.js.
ckeditor/plugins/mwtemplate/plugins.js has code for double click of element "class = FCK__MWTemplate"
but for some reason it is not activated there on doubleclick of icon_template.gif,
placing similar code in ckeditor/plugins/mediawiki/plugin.js seems to solve the case.