If you're interested I'm doing this myself and it'll be checked by MyBB SQA, and then I'll release it and it'll be used on the MyBB forums (that's why SQA are checking it). It's looking good so far, should meet people's needs. For putting [solved] in the title, I wouldn't actually edit the thread title, just display [solved] before the threads, so it does the same thing but doesn't interfere with the thread itself and is easily removable. Better way of doing it IMO.
MyBB Support Team Lead