I have a zyxel 2x router/FW that I would love to replace with a Mono
box, but I host about 15 domains inside the NAT router, including DNS,
mail, www, etc and I can access mydomain.com from inside or outside
with the zyxel.

It seems to me I can't use the dns forwarder feature of Mono due to a
couple of reasons.  I use AD so I have ( ? ) to use the DNS of my
domain controller, and secondly, the domain forwarder will just route
my requests to a certain IP, so I lose the header info, which many of
the  servivces use to figure out what to do...  i.e. the multiple
websites...

I install Mono Boxes wherever I can, but unfortunately I can't have
one myself.  I don't plan on setting up 2 DNS servers.  The zyxel has
some shortcomings that frustrate me.  In general I love Mono, but this
loopback issue is a showstopper for me, and it appears, for many
others too.