Why not give them to senior members? I assume the reason that you're not just letting every member tag threads is that you can't trust every member to do it properly. Doesn't being a senior member imply some level of trust in the person? The forum has over 28,000 threads and the staff has only managed to tag a small few of them. Even disregarding tagging, it's clear you have a large work load, as you recently implemented sub-mods to help take the load off. Of course, it wouldn't have to be the job of senior members, but I'm sure they would all tag a few threads every now and then.
NOTE: If the seniors already have permission I apologize for making this thread.