Support's statement is correct and there isn't really a workaround because it simply hasn't been implemented in Workspaces. But it is worth clarifying what "Optimistic locking" provides you as this should stop data being overwritten:
1. User A opens an incident in Workspaces.
2. User B opens the same incident in Web Desk.
3. User B makes changes and saves.
4. User A makes changes and saves - if any of the changed values were also changed by User B the save fails and an error message is displayed.
For a lot of customers this level of locking is enough and sometimes more appropriate than "Optimistic with warning". I don't know when/if "...with warning" is due to be implemented but you could log an ER.
(If the above steps don't work get that logged with Support!)