<p>We have an home-brewed XMPP server and I was asked what is our server's MSL (Maximum Segment Lifetime).<br>
What does it mean and how can I obtain it? Is it something in the Linux <code>/proc</code> TCP settings?</p>

This looks like it can answer your question:
http://seer.support.veritas.com/docs/264886.htm
I suggest that you ask why someone asked you this and find out how that applies to XMPP.
TCP/IP Illustrated volume 1 is online and describes 2MSL in more detail:
Here
MSL is also described in the TCP
RFC 793
as mentioned in
wikipedia