I work on 
OpenFilesListPlus plugin, an enhanced version of 
OpenFilesList plugin.
- Each project stores which sub-panel each of its files belongs to
- The workspace stores the sub-panels layout
For saving the sub-panels layout, I have to detect the workspace closing time. 
Unfortunately I did not succeed in this using the existing cbEVT_WORKSPACE_LOADING_COMPLETE and cbEVT_WORKSPACE_CHANGED events.
Adding the cbEVT_WORKSPACE_CLOSE_BEGIN appears to be the best way for achieving my goal.
And incidentally it is only a few lines of code, so please add 

Regards ~