I think this is another feature which needs to be added, like those asked for above.
For the time being you can work around it (as with most moderation powers, like edit, pin, close, split, etc.) by bumping up your category-specific moderators to TL4. If you don’t want to make them all TL4 because of the other powers they get (across the entire forum), you can lower the trust level required to mark a topic solved with the
accept all solutions trust level setting.