Before we get to the how though we have to decide whether. If this is
going
to be practical and usable it has to be widely supported.
I think it's worth pointing out that it's probably going to be difficult to
apply per-user to HELO.
But that aside, I could imagine that organisational units having special
needs, might publish research.example.com, sales.example.com and these can
be included in the top-level record. How to maintain sub-domains per-user
(by further inclusion?) becomes a problem for the sub-unit. Which is as it
should be, perhaps.