Until we have an XML catalogue system, could you register the DTD with
the SGML catalogues? (and of course hack fonts.conf so refer to the
DTD correctly)

Just to update this bug report, we now have an xml catalog , i
have looked at docbook-xml to make a few guess and :
http://debian-xml-sgml.alioth.debian.org/xml-policy/xml-catalogs.html

It seems dh_installxmlcatalogs should manage the biggest part of
the job.
docbook depends on xml-core and sgml-data which seems need for
any xml dtd . And also sgml-base though i don't know if fonts.dtd
should be registered to sgml catalog too (dh_installcatalogs).

As fontconfig never registereded to those catalogs this should be
enough.

This is merely more than a wishlist as the xml policy is only a
WD by now. It build on the previous sgml work so should be pretty
stable from the start.

Regards
Alban