Dear all,
The squid 2.4 equivelent of "header_access Accept-Encoding deny all" is "anonymize_headers deny Accept-Encoding", but it lacks the feature in squid 2.5 of limiting to this option to be applied to hotmail, i.e. this option applies to ALL proxy requests.
I have tested this option for squid 2.4.stable6 and it worked for me.