Header (not envelope) rewriting in zmailer?

[Arnt Gulbrandsen <agulbra@troll.no>]

I need to block mail from outside this site to certain aliases.  The
aliases mostly point to other sites.

I can see two ways to do it:

 - hide the addresses.  Rewrite the outgoing To: header to something:;
   and assume that nobody outside will guess what the magic alias is.

 - do something crossbar-like, but before alias expansion.  crossbar()
   at present doesn't see the alias, so mail from outside to these
   aliases looks like ordinary transit mail inside crossbar().

However, I can't see where I can put in these hacks.  Is it at all
possible?

--Arnt