I just notice that OnPreSave / OnSave / OnPostSave methods are called wether the user trigger a standard save operation or when Sketchup save the file through AutoSave method.
I can’t find a way to differentiate the calls and find out if it is a user operation or an autosave operation.
This is an excellent workaround that nearly does the trick.
Anyway a problem is remaining.
When saving for the first time on a newly created scene that does have name, model.path remains empty in onPostSave methods.
The filename is attached to the model after the onPostSave method call, which is definitively a strange behavior. onPostSave should access to the details of the save operation as it has been completed.
So this workaround only works from the second save, where the model.path is correctly defined.