In CMIS there is no way to fulfill your goal. spec indicates that all version control properties on cmis:document are read-only, and the only way to influence them in some way is through the version API, which allows you to specify whether you will create a small or major version and an optional comment for verification.
You will need to write an even more complex server-side extension for Alfresco if you want to do this, or expand your content model to provide a custom property for storing the information that you now want to save in the version label.
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