I've done something similar for Risks & Issues. Essentially I created a custom field that gets populated with whatever the last discussion message was.
I've created a workflow rule for "Discussion message" called "update last update" which updates a custom field for that I have created for cases called "Last Update"
The workflow rule is the following:
This should work for you if you change the "Risk" & "Issue" to Project and specify which field of the Project you want to update (or create a new custom field).
Be careful that the various text fields can have different formats. That's why I have chosen to create a new custom field which is a rich text field (Same as the discussion message)
This also includes any replies to discussion messages, if you don't want that only create the workflow rule on Post rather than Discussion message.
Hope this helps