Peter, your questions so very often require taking another look at what is taken for granted and makes us question what is the real intent.
One thing stands out with the Code - the definition of Principal says nothing about former clients or employer's of the Member.
Also, the definitions speak to the Member providing to the Principal "retirement plan services" but this term is not defined in the Code of Conduct.
You suggest assuming the work is Professional Services. The Code does define "Professional Services: services provided to a Principal by a Member, including the rendering of advice, recommendations, findings, or opinions related to a retirement or other employee benefit plan."
Professional services as defined in the Code are vastly different from the services provided by many employees of a recordkeeper where the employee's job is to follow a fixed set of administrative steps with no rendering of advice, recommendations, findings, or opinions related to a retirement or other employee benefit plan is involved.
The Code also notes that "A Member shall render opinions or advice, or perform Professional Services, only when qualified to do so based on education, training and experience." People in our industry more often than not try to help out a participant that is struggling with understanding plan provisions and available options.
To quote Dirty Harry, "a man has got to know his limitations."