As far as I know, ant loglevel has been available since Buckminster 3.7 (thus, some time ago), so we might add a help note saying that at least such version is required in order to use such option. Concerning a free-form "Buckminster Options", that's already available: "Equinox Launcher Arguments", see also http://stackoverflow.com/questions/18058279/buckminster-3-6-and-above-on-jenkins-limited-error-feedback to see how to specify "--loglevel ant=debug" in the current version of the Buckminster plugin.
I'd like to try to add the additional dropdown list (probably in the next few days), or if you have time and you want to try :)