How can I make squid add its own http header to the reply, based on ACL?

Solution 1:

Have you taken a look at the request_header_add directive? It was only just added in release 3.3.

See the directive use here.

From the text;

One or more Squid ACLs may be specified to restrict header
injection to matching requests. As always in squid.conf, all
ACLs in an option ACL list must be satisfied for the insertion
to happen. The request_header_add option supports fast ACLs
only.

I'm not sure if that is what you're interested in, or if you want to append headers to the response and not the request. Squid did not previously have support for header addition with ACL, only modification or removal I believe.