It is possible to add on the site, however I've tried an addon that does the same thing - but it has some compatibility problem as it means we can't use the built in tag system for other purposes. Editing the titles to include the solved tag manually also comes with problems, as it takes up a lot of moderator time and it also reduces the relevancy of the threads in the eyes of search engines (perversely, given the fact it should help!).
For the time being I'm not going to add this feature, but if there is an improved solution in future then I'm happy to revisit it
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