How to set or clear a Rich Text Commerce attribute in a mass update

Summary:

We need to set or clear a Rich Text Commerce attribute (myRTattribute) in a mass update. But a simple "1~myRTattribute~dummy|" doesn't work.
Even triggering an action via webservice during the mass update (the action setting the attribute in the same way) doesn't work.
How can we set a rich text attribute in a mass update?

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