To be honest, it could be done but it would take a long time to do it as each post would have to be done individually by admin or mods unless the original posters did that.
The best thing to do is read the first post as that should tell you straightaway when is it etc.
This is also the reason why its important for people to correctly label/ put date & location in the title but its not always done
I think that you could ask the original posters to change the title afterwards. It wouldn't be hard and they could add a note on how it went
Just adding " MEET FINISHED " would mean you could still add post on what happened but poeple wouldn't like Gixer open the topic to find it over.
Putting the date on the topic title may help but if the titles long this may be missing when is shown on forum page !?
The Calendar useful if poeple used it