Enrico,

My first answer may need clarification. In fact there were two questions 
in your mail:

#1 - How do I make the WAN interface responds to ARP queries for 
additionnal IP address?

#2 - How do I set up NAT for the DMZ services?

>>Proxy ARP is your friend.
>>

I admit I've focused on #1 and simply forgot #2 :-(

I dont know how you get this 2 IP addresses, but in most cases you don't 
need Proxy ARP and I think 1:1 NAT is what you need.

>>Read the Doc!

A more useful answer would have been:
http://www.m0n0.ch/wall/docbook/examples.html#id2598130


-- Vincent