It might not be a perfect solution, but bear in mind that you can post a brief topic in another group, pointing at your original comment. See my pairs of comments:
Whilst you can’t search by tags yet, you will be able to by the end of Summer, so you can tag your posts to show they fit several categories.
We could reinstate the tag cloud after searching has been improved. There is an Elgg plugin – http://community.elgg.org/pg/plugins/steven/read/384276/tagcloud-plugin.
Would other users like the tag cloud?