Actually, the advanced widget does support filtering on custom taxonomies as I tested it out using the widget itself.
I also just finished editing the plugin to get it working with a new parameter. I basically just duplicated all the plugin code I could find that contained the post_tag logic and edited it to use event_tag also. It seems to be working just fine. Not the most elegant solution but it’ll get me by until this is supported natively in the events calendar plugin.
Thanks again for your help and quick replies.