I think this is an interesting idea. However I am not a fan of the chosen way to display the broken link(s). In a discussion it could be multiple links that are broken. Also it should be easily visible to the user. What I'd do (don't know if it's possible though) is render the link non-clickable and add
(broken link) inline.
How would checking every link in a discussion impact performance of loading it? When is a link broken?