If you are happy with a two stage process then I guess you could add a tag to the badge winning topic and then target that tag and add the badge with a mix of jQuery and CSS or just CSS if you placed the badge in the same Dom fragment as the tag. Note, with CSS you can add text and graphics only.
A nice bonus would be that you could list all, for example, gold star topics by selecting the gold star tag.
Edit. Couldn’t you also start by tagging the topic and then award the badge to the pupil by badge script based on the topic having that tag? Wouldn’t that one tagging action create a badge on the topic and award it to the pupil too?