Here is an amended request that could accomplish what I want, while separating my request from the debate about what the SUBJECT line should be.
All I really would like to see is an attempt to mimic normal email where the sender (or recipient, in the case of sent email) is visible and sortable in the list of emails in ones email program. Now in the case of Wikitree, sent messages are copied to oneself, and thus both received and sent emails (to another Wikitree member) appear in ones inbox, along with received messages from non-Wikitree members.
Technically I believe that it is quite possible to set the FROM field in an email to be whatever the programmer wants. Right now Wikitree already does this where some emails that say from Wikitree are from two different actual email addresses: firstname.lastname@example.org & email@example.com. Also, there are some messages from firstname.lastname@example.org that are labelled as Wikitree and others as Wikitree G2G.
Thus an alternative to what I requested above would be to make the FROM field as follows:
from or to a Wikitree member:
Surname-NNN via Wikitree
from a non-Wikitree member:
email@example.com via Wikitree
rather than just saying Wikitree.
In this manner it changes nothing about what the SUBJECT line can be, but provides the clarity about who the conversation is with.
P.S. note that for communications between Wikitree members, the Surname-NNN is for the other party. Thus on a sent message, the copy sent to the inbox of the Sender would be labelled Recipient-NNN, while the copy sent to the inbox of the Recipient would be labelled Sender-NNN.
P.P.S. note that there are many messages that listed as just being from Wikitree that would be usefully tagged in this manner, i.e. any message sent to one member due to the actions of another. Why not identify the other member right in the FROM field of the email, so that communications from a particular other member can readily be grouped for review.