Unsupported, but this works :)
You can modify the aspx file, which you want to change the behaviour of the save button. Insert this script into the file and you will get an alert before saving the file:
//make the form aware of our new save method
crmForm.onsave = overriddenSave;
crmForm._bUseCustomSaveEvent = true;
//implementation of the new save method
//do your thing here
alert('Your modifications will be saved now');
//save the form.
crmForm.SubmitCrmForm(1, true, false, false, false);
Hope this helps!