The ideal would be to include the asterisk in a custom field and have it included (maybe with a tab following). Not sure how you can get it “hanging” though unless you lead with <custom field>tab (for each ref type template that will be using in your bibliography) then the rest of the template as hanging indent (set in the layout section at the very bottom right side). Don’t use the separator characters cause in this case you will want every reference to be indented.
But if you don’t use the “label” field for anything else, you could put your * in that field, and then adjust the style’s bibliographic “layout” setting to include the Label field (insert Label) and then insert tab, and make sure the hanging indent is set there too (as above).
haven’t really tested either. just brain storming.