Yes, I am using the Local User Manager for authentication, with the users added at the System/User Manager menu.
However, I have only the first box checked. The second is not checked.
This is the opposite of what you recommend, although the result appears to be the same (ie. after login the user mac address gets added to the passthrough list with a message like 'auto added mac for user bobby').
Perhaps PfSense has a fallback mode whereby it includes the username when it is present.
Anyhow, I will try switching the checkboxes at some convenient quiet time and experiment some more with a view to finding any potential subtle differences or misunderstandings.
If I spot anything I will write again.
In the meantime, thank you again for your help.
Michael.