In response to above, no, presently there is not clean support for that.
  See this bug:
http://www.nsnam.org/bugzilla/show_bug.cgi?id=188

We are reworking the Ipv4Interface class to allow it to hold multiple
IPv4 addresses (one primary and multiple secondary), as in Linux, but
that support will not be merged until after ns-3.3 is released.  The
code that implements this IP aliasing is presently in the private branch
http://code.nsnam.org/tomh/ns-3-ip

Regarding your other question:

 > My personal observation is that  the same type of symptom (lack of
 > routing) is same for three different (separate) type of
 > configurations:
 > 1. Adding an extra loopback (as described above)
 > 2. Adding second IP address to the same interface (this is unsupported
 > as you pointed out Mathieu)
 > 3. Adding a sort of pseudo-interface (additional csma or p2p) to act
 > as loopback and addressing it with /32 (as I described in my initial
 > email).
 >
 > All three scenarios are failing in the same way - routing is broken.
 >

I believe that this in general is related to the lack of support for
stub networks, as has been recently called out in the end of bug 406:
http://www.nsnam.org/bugzilla/show_bug.cgi?id=406

I believe that, after bug 406 is fixed, you should be able to create a
new virtual device (a second loopback) and put an address on it and use
that as a router ID; I'll write a test case for that scenario.

I am going to prioritize fixing the above this week and will get back to
you when I make progress.