How do you update an External Data column in a SharePoint list item?

[bradleyl]

We have an External Data column – "QVAS"  which is linked to a table in a SQL server database  ( the database is called QVAS)

We can update the item fine using SharePoint forms with the usual select or query options available.

The column stores the key to some property ownership data , and the related fields are important , e.g. QVAS:Owner and QVAS:LGA  . See below

However, we need to use Nintex workflow to update this column. But we cannot workout how to do this using the Nintex workflow's "Updating Item" action in a workflow

because 

• Only the External Column ‘QVAS’ is available to update
• None of the “related” columns e.g. ‘QVAS: Owner’ or ‘QVAS: LGA’ are available to write to
• When we write the Key value into the ‘QVAS’ column, as a string,  it does appear,  but is NOT linked to the External Content Type, so “related” columns all show as blank
• So the column shows in View item, but Not in Edit Item (the column is blank)
• The Hidden column related to the external Column, (probably ‘QVAS_ID’) is not visible in the Nintex "Update Item" action and so cannot be written 

Question How can we update SharePoint External Data Columns using Nintex Workflow?

Is it possible?

Is it possible using a Web Service Call, perhaps using REST , or SOAP?

We can use the Query BCS action to get values from the BCS, but cannot write them to the related list.