Dano,

Perhaps this could go so far as to allow the owner to change message
delivery settings and add alternate email addresses.
...
I recognize this pushes some security concerns ...

Yeah, no kidding.

In Yahoo Groups the "Alternate Posting Address" was treated as an alias for the member's true address. In particular, without looking at the message header one couldn't tell that the alternate address had been used. I think under the hood it operated a lot like an email From address spoof. In the hands of an unscrupulous moderator that could be a nasty way to damage a member's reputation or convert the member into a sock-puppet.

If Groups.io were to implement something like this, or the ability to change a member's email address, I think it might be best thought of in the same ways as a Remote Assistance session: very useful, very dangerous.

I think member-initiated and time-limited are two good restrictions. My primary concern with that is the one expressed by Dano and Judy: how to make it simple enough for the member who needs it most to initiate it. Maybe there needs to be more than one way: an email way for members that can't find the group's pages and a web way for those who can't fathom an email command.

Direct add of members would allow us some way to reconnect him and
delete his old account, but we still need to be able to change things to
help him out.

That's the only way to change the address of an email-only member: Remove the old address and Add the new one. Which in Yahoo Groups ceased to be possible when they completely removed the Add Member feature. And isn't possible for free in Groups.io.

For members with web access that method is rude: it discards their subscription settings and destroys their identity as the owner of prior content (they would no longer be able to delete or edit the messages posted under the old address).

-- Shal