I just tried your suggestion and squid still isn't listed in menuconfig although it should have been included via packages:
$ ./scripts/feeds search squid
Search results in feed 'packages':
crowdcontrol The extra-light http proxy server
squid full-featured Web proxy cache
squid-mod-basic-auth-getpwnam getpwnam basic authentication helper
squid-mod-basic-auth-ncsa NCSA basic authentication helper
squid-mod-basic-auth-smb Samba basic authentication helper
squid-mod-cachemgr Web based proxy manager and reporting tool
squid-mod-digest-auth-password Password digest authentication helper
squid-mod-external-acl-ip-user IP user external ACL helper
squid-mod-external-acl-unix-group Unix group external ACL helper
squid-mod-ntlm-auth-fakeauth Fakeauth NTLM authentication helper
squid-mod-ntlm-auth-smb-auth Samba NTLM authentication helper
wccpd a Web Cache Coordination daemon
Ignoring feed 'xwrt' - index missing
packages revision 16650, luci revision 4961. Searching using '/' in menuconfig still lists squid as being under Network->Squid.
I do see that squid has been downloaded and placed in ~/trunk/feeds/packages/net/squid, perhaps a symlink is missing? Any ideas?