In fact there's even a suggested protocol for it. I don't recall the draft
name, but it works by putting the address list in the DNS. You then use zone
transfer to move the data around and keep it up to date.
Remember the reference? I was thinking of something like that, but if
it's already done, all the better.
There are also at least two, and probably more, "lookahead" milters that use
part of an SMTP transaction to perform early address validation. (I use one of
these myself as part of providing backup MX service for a couple of people.)
I understand about callforwards, but it seems to me that the backup MX
is mostly going to get mail when the main MX isn't available, so I
don't see how this would work very well in practice.
R's,
John