Yes, LimeLM supports tagging product keys. The only caveat is that these tags aren't visible in the UI (as of the time I'm writing this). We plan to make tagging visible in the UI, but it will take a bit of work to make it good.
However, the web API has fully supported tagging for a couple of years now. For example, to set tags, call limelm.pkey.setTags. And to search based on tags you can use limelm.pkey.advancedSearch.
Tell me if that helps.