Why do you hate port 80?
The best way to unify the name for many hosts is by DNS on all the hosts, so the DNS nameserver rotation can find the host that is up and it gives only it's own address for the shared name. However, you have one host.
If you turn on routing software, packets to A can be routed to b and vice versa, so you can use either. Any host with two ip ports and ip forwarding is a router, but to be know, it needs to have route publishing software and be accepted by routers.
You could write a widget for port 80 that redirects using the IP or host name of that side. C and perl and inetd all are happy to listen, and the redirect dialog is rather simple. It might already be out there, for many domains have the domain name as a host name for the mail portal, and redirect all port 80 calls there to the web server. If you send the moved permanently http response, clients should update their bookmarks and such.
Redirecting ports is just a side issue, once you have a unified name:
http://httpd.apache.org/docs/1.3/mod/mod_alias.html#redirect
Maybe we are working to hard on redirect. Two of these Apache directives might do the trick (status should be permanent to send 301 and update clients):
RedirectMatch [status] regex URL
