Exception :

Column 'ClientAccountName' does not belong to table mt_pch_companies.

1. The custom property is an actual field in the table that is expected when updating in which case you would want to add the field to the ExcludeOnUpdates property.
2. You are evaluating the field (referencing the field) when you have not pulled back the field that is used by the custom field
3. Calling the MyBo.Item property to retrieve the vaue from within that item which would cause a recursive error in which case you should be using MyBo.CurrentRow.Item("MyField")