It would be useful if the Squid package offered a Debian-friendly way to
replace error text (beyond making a custom package that Provides squid-langpack
or using dpkg-divert to protect my changes). I have an situation where I must
alter the error text, and the files in /usr/share/squid3/errors aren't
conffiles.

There are three methods of server-side text replacement available
through squid.conf (deny_info for targeted responses,
error_default_language for custom page names, error_directory for
complete page set replacement) and client-side replacement via CSS in
/etc/squid3/errorpage.css configuration file.

What is your particular use-case which these do not solve?

Amos
Squid Project