You could add a Message step of type "Webhook" to call Braze /users/track API to attribute a custom user attribute, and then use this attribute for the Decision split block.

In the Webhook you have access to canvas entry properties which you can access like this:

{
  "attributes": [
    {
      "external_id": "{{${user_id}}}",
      "custom_property": "{{canvas_entry_properties.${custom_value}}}"
    }
  ]
}

and when triggering the Canvas via API passing it to the canvas_entry_properties like this

{
    "canvas_id": "<canvas id>",
    "recipients": [
        {
            "external_user_id": "<user id>",
            "canvas_entry_properties": {
                "custom_value": "Value"
            }
        }
    ]
}