BUT HERE IS THE KICKER. The management company rep says that it is
*illegal* to tell the tenant what is about to happen to him. That
they can't tell him this without the consent of the landlord. She says
she has verified this with a rental type lawyer, the HOA's lawyer,
recently, but she knew it before that.

So the HOA itself cannot *officially* notify the tenant. As noted,
there's nothing stopping anyone else from unofficially taking matters
into his/her own hands - as long as s/he isn't acting on behalf of the
HOA.

I seriously doubt that this is true. If you are a utility provider,
(which you apparently are, if you can shut off the water
and not just ask the water co. to do it), then you can certainly
tell the occupants that you're going to do so, just as if it were
a normal maintenance notice. When they ask why or for how long,
you then say "your landlord has the details, talk to them."