Well it wasn't so easy when the topic came up about access to certain forums as that required group access, e.g. forum status and not by individual and this seems very similar.
Yes, you can make individual forum members moderators, thereby giving them modification status, but I doubt this can be set against a member that doesn't have moderator status. That however, is speculative/logical thinking as I don't have access to the forums software settings.
I know one thing though, I don't see the problem if a topic is marked solved or not so long as anyone viewing it later will see the solution. Also as the topic is still open others may subsequently post in it (seen in many old topics), so unless said marking also locked the topic I don't see the point.