I'd like to make sshd use different host keys on different ports...

We have only one public IP address, so we used different port numbers (22,
23, and 24) to reach multiple servers (xxx, yyy, and zzz respectively)
beyond the front one (xxx) directly fromt he Internet.

We are consolidating these servers into one (iii) now and would like it to
listen on all three ports using the old servers' keys so as to be as
transparent to the users as possible.

I was hoping, something like

Port 22
HostKey /etc/ssh/ssh_host_key-xxx

Port 23
HostKey /etc/ssh/ssh_host_key-yyy

Port 24
HostKey /etc/ssh/ssh_host_key-zzz

would work, but it does not -- the server uses the first one
(ssh_host_key-xxx) on all ports...

Any hope? We could, of course, run three independent sshd processes, but
that's ugly...

Thanks for ideas!