Answer
No — data entered across an assistant/wizard's steps isn't physically persisted to the database until the assistant actually completes; it's purely transient client-side state used to drive the assistant's own flow. Because of this, there's no database-level hook point for a Custom Event to intercept or react to mid-assistant, at any step before the final commit — any validation logic needed between steps has to be built into the assistant's own step configuration rather than via a Custom Event.